Syntaxes not yet supported in `vim-just`

[laniakea64]

• const expressions in settings [1], [2]
• long keyword without value in [arg] attribute
• [env(NAME, VALUE)] attribute
• [dragonfly], [freebsd], and [netbsd] attributes
• set guards and ? guard sigil
• set lazy
• eager keyword
• Overriding variable defined in a submodule
• runtime_directory() function
• Expressions in attribute arguments
• User-defined functions
• comment after unexport statement
• module_path() function
• [android] attribute
• set no-cd

[laniakea64]

Sorry for dropping the ball on keeping up with this in the last few months, unfortunately I'm too swamped to be able to work on this in the foreseeable future.

Tagging as good first issue Good for newcomers since some syntaxes on the list, notably the new settings & new attributes, are about as straightforward as supporting syntaxes in vim-just gets and previous similar commits could likely serve as partial guides of some aspects of how to do it. However the others look likely more involved at a glance (some e.g. the long keyword will even require rethinking existing highlighting)