This commit adds standard conventions for Vim filetype plugins:
- Allows users to opt out of using the provided ftplugin file, if they
choose to install and use another set of runtime files for Swift
(which offers its own version of ftplugin for Swift), It also prevents
this ftplugin file from being sourced again if a ftplugin file for
Swift was already sourced. Vim's documentation on this recommends
offering users this option as well, under the "DISABLING" section
of `:help write-filetype-plugin`).
- Adds the `b:undo_ftplugin` buffer local variable, which is used to
undo the filetype settings when the `:setfiletype` command is used
(See :help `undo_ftplugin`). Also prefer using the full names for Vim
settings instead of short ones as they are more readable.
The above conventions are in place in many of the ftplugin files
shipped with Vim, so they can be used as a reference, as well.
