runtime(algol68): Add new syntax file, ftplugin and filetype detection

- Add a syntax file update to Neville Dempsey's long-serving version
- Add a new rudimentary ftplugin
- Add filetype detection

Changes to the syntax file include:
- improved prelude, number and symbol highlighting
- prelude highlighting tests
- updated boiler plate

Note that these runtime files currently target Algol 68 Genie employing
the default UPPER stropping regime.  Support for GNU Algol 68 should
also be usable with the UPPER stropping regime, although somewhat less
complete.  Full support for the SUPPER stropping regime in GNU Algol 68
is also planned.

closes: #19818

Co-authored-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Janis Papanagnou <janis_papanagnou@hotmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/testdir/input/algol68_denotations.a68

@@ -0,0 +1,115 @@
+# Algol 68 Denotations #
+
+
+# BOOL #
+
+BOOL x = TRUE
+BOOL x = FALSE
+
+
+# INT #
+
+INT x = 0
+INT x = 42
+
+# whitespace #
+INT x = 1 000 000
+
+# monadic operators #
+INT x = -42
+INT x = +42
+
+
+# REAL #
+
+REAL x = .42
+REAL x = 42.42
+
+REAL x = 42e42
+REAL x = 42E42
+REAL x = 42\42
+REAL x = 42⏨42
+
+REAL x = 42e-42
+REAL x = 42E-42
+REAL x = 42\-42
+REAL x = 42⏨-42
+
+REAL x = 42e+42
+REAL x = 42E+42
+REAL x = 42\+42
+REAL x = 42⏨+42
+
+REAL x = .42e42
+REAL x = .42E42
+REAL x = .42\42
+REAL x = .42⏨42
+
+REAL x = .42e-42
+REAL x = .42E-42
+REAL x = .42\-42
+REAL x = .42⏨-42
+
+REAL x = .42e+42
+REAL x = .42E+42
+REAL x = .42\+42
+REAL x = .42⏨+42
+
+REAL x = 42.42e42
+REAL x = 42.42E42
+REAL x = 42.42\42
+REAL x = 42.42⏨42
+
+REAL x = 42.42e-42
+REAL x = 42.42E-42
+REAL x = 42.42\-42
+REAL x = 42.42⏨-42
+
+REAL x = 42.42e+42
+REAL x = 42.42E+42
+REAL x = 42.42\+42
+REAL x = 42.42⏨+42
+
+# whitespace #
+REAL x = 4 2 . 4 2 e + 4 2
+REAL x = 4 2 . 4 2 \ + 4 2
+REAL x = 4 2 . 4 2 ⏨ + 4 2
+
+# monadic operators #
+REAL x = -42.42
+REAL x = +42.42
+
+
+# BITS #
+
+BITS x = 2r101010
+BITS x = 4r222
+BITS x = 8r52
+BITS x = 16r2a
+
+# whitespace #
+BITS x = 2r 1010 1010
+
+# unmatched by bad pattern in legacy versions of syntax file #
+BITS x = 16r99
+
+
+# STRING #
+
+STRING s = "Foo Bar"
+
+# quote escape #
+STRING s = """"
+STRING s = " "" "
+STRING s = "Foo"""
+STRING s = """Bar"
+STRING s = "Foo""Bar"
+
+# line continuation #
+STRING s = "Foo\
+ Bar"
+STRING s = "Foo""\
+Bar"
+STRING s = "Foo\
+""Bar"
+