support modelines

[mcandre]

For identifying and skipping implementation-specific makefiles.

Emacs file local variable:

# -*- mode: <language> -*-

where <language> is a make language variant, such as make, gmake, bmake, nmake, etc.

Vim modeline:

# vim: ft=<language>:

We should confirm the exact language names in both Vim and Emacs, in case they require subtly different names for the various make implementations.

Vim users also apply instead the bindings syntax or filetype.

See also the polyglot forms:

# -*- mode: <language>; -*-
# vim: set syntax=language:

# -*- mode: <language>; -*- vim: set ft=<language>:

https://marketplace.visualstudio.com/items?itemName=chrislajoie.vscode-modelines

We should warn to encourage this mode line in *.include.mk files.

We should document in the parsing guide that mode lines are good to use, especially for makefiles using non-POSIX extensions.

[mcandre]

Pending ctlajoie/vscode-modelines@8a6db4d

[mcandre]

Oh wow, POSIX vi dropped support for modelines.

All the more reason to use ft rather than syntax in Vim and Emacs modelines.