runtime: Update files for ConTeXt, METAFONT, and MetaPost.

This update is meant to be included in the upcoming 9.2 release.

**New**

- Support ConTeXt's convention to optionally specify an output directory
  in a comment line at the beginning of a source file.
- If a log file is not found, Vim does not create a new buffer.
- Removed `syntax/shared` files for the following reasons:
  - they are not necessary for the plugin to work (they only improve
    over existing syntax highlighting);
  - they are relative large;
  - they can be automatically (re)generated by users at any time using
    ConTeXt (explained in the doc);
  - since ConTeXt is updated frequently, they quickly become obsolete.

**Minor**

- Prefer `var` to `const` inside functions.
- Prefer `$`-interpolation to `printf()`.
- All revision dates set to the same date for consistency.
- Updated the error format.
- Various tweaks to the documentation, but nothing disruptive or new.

closes: #19148

Signed-off-by: Lifepillar <lifepillar@lifepillar.me>
Signed-off-by: Christian Brabandt <cb@256bit.org>

.github/MAINTAINERS

@@ -10,6 +10,7 @@
 # will be requested to review.
 
 nsis/lang/russian.nsi					@RestorerZ
+runtime/autoload/context.vim				@lifepillar
 runtime/autoload/freebasic.vim				@dkearns
 runtime/autoload/hare.vim				@selenebun
 runtime/autoload/hcl.vim				@gpanders
@@ -17,6 +18,7 @@ runtime/autoload/javascriptcomplete.vim				@jsit
 runtime/autoload/modula2.vim				@dkearns
 runtime/autoload/rubycomplete.vim			@segfault @dkearns
 runtime/autoload/rust.vim				@lilyball
+runtime/autoload/typeset.vim				@lifepillar
 runtime/autoload/xmlformat.vim				@chrisbra
 runtime/autoload/dist/json.vim				@habamax
 runtime/colors/blue.vim					@habamax @romainl @neutaaaaan
@@ -48,6 +50,7 @@ runtime/colors/zellner.vim				@habamax @romainl @neutaaaaan
 runtime/compiler/biome.vim				@Konfekt
 runtime/compiler/checkstyle.vim				@dkearns
 runtime/compiler/cm3.vim				@dkearns
+runtime/compiler/context.vim				@lifepillar
 runtime/compiler/cucumber.vim				@tpope
 runtime/compiler/dart.vim				@dkearns
 runtime/compiler/dart2js.vim				@dkearns
@@ -142,6 +145,7 @@ runtime/ftplugin/chicken.vim				@evhan
 runtime/ftplugin/clojure.vim				@axvr
 runtime/ftplugin/cmakecache.vim				@ribru17
 runtime/ftplugin/codeowners.vim				@jparise
+runtime/ftplugin/context.vim				@lifepillar
 runtime/ftplugin/cook.vim				@ribru17
 runtime/ftplugin/cs.vim					@nickspoons
 runtime/ftplugin/csh.vim				@dkearns
@@ -241,9 +245,11 @@ runtime/ftplugin/markdown.vim				@tpope
 runtime/ftplugin/mbsync.vim				@fymyte
 runtime/ftplugin/mediawiki.vim				@avidseeker
 runtime/ftplugin/meson.vim				@Liambeguin
+runtime/ftplugin/mf.vim				@lifepillar
 runtime/ftplugin/modula2.vim				@dkearns
 runtime/ftplugin/modula3.vim				@dkearns
 runtime/ftplugin/mojo.vim				@ribru17
+runtime/ftplugin/mp.vim				@lifepillar
 runtime/ftplugin/mss.vim				@Freed-Wu
 runtime/ftplugin/nginx.vim				@chr4
 runtime/ftplugin/nim.vim				@ribru17
@@ -346,6 +352,7 @@ runtime/indent/cdl.vim					@dkearns
 runtime/indent/chatito.vim				@ObserverOfTime
 runtime/indent/clojure.vim				@axvr
 runtime/indent/config.vim				@dkearns
+runtime/indent/context.vim				@lifepillar
 runtime/indent/cs.vim					@nickspoons
 runtime/indent/css.vim					@dkearns
 runtime/indent/cucumber.vim				@tpope
@@ -392,8 +399,10 @@ runtime/indent/lua.vim					@marcuscf
 runtime/indent/m17ndb.vim				@dseomn
 runtime/indent/make.vim					@dkearns
 runtime/indent/meson.vim				@Liambeguin
+runtime/indent/mf.vim				@lifepillar
 runtime/indent/mma.vim					@dkearns
 runtime/indent/mojo.vim					@ribru17
+runtime/indent/mp.vim				@lifepillar
 runtime/indent/nginx.vim				@chr4
 runtime/indent/nsis.vim					@k-takata
 runtime/indent/nu.vim					@elkasztano
@@ -474,6 +483,7 @@ runtime/syntax/chicken.vim				@evhan
 runtime/syntax/chuck.vim				@andreacfromtheapp
 runtime/syntax/clojure.vim				@axvr
 runtime/syntax/codeowners.vim				@jparise
+runtime/syntax/context.vim				@lifepillar
 runtime/syntax/cs.vim					@nickspoons
 runtime/syntax/css.vim					@jsit
 runtime/syntax/csv.vim					@habamax
@@ -580,11 +590,13 @@ runtime/syntax/mbsync.vim				@fymyte
 runtime/syntax/mason.vim				@petdance
 runtime/syntax/mediawiki.vim				@avidseeker
 runtime/syntax/meson.vim				@Liambeguin
+runtime/syntax/mf.vim				@lifepillar
 runtime/syntax/modula2.vim				@dkearns
 runtime/syntax/modula2/opt/iso.vim			@trijezdci
 runtime/syntax/modula2/opt/pim.vim			@trijezdci
 runtime/syntax/modula2/opt/r10.vim			@trijezdci
 runtime/syntax/modula3.vim				@dkearns
+runtime/syntax/mp.vim				@lifepillar
 runtime/syntax/mss.vim					@Freed-Wu
 runtime/syntax/n1ql.vim					@pr3d4t0r
 runtime/syntax/nginx.vim				@chr4