Experimental Vim9 support

Not planning to merge this, just playing around. Changes were mostly mechanical, so it wasn't a big deal to apply. I might do something like keep both versions of the code in separate files and if-clause them, which might actually make it mergeable.

Performance-wise, I tested on a particularly annoying edge case, a list of 199 entries that looks like this:

[
  'Test',
  'Test',
  # ...
  'Test',
]

Every indent needs to find the "most significant line" with a linear search, going through every line above until the [. Anyway, here's a result of profile func GetRubyIndent on master:

FUNCTION  GetRubyIndent()
    Defined: ~/.vim/bundle/ruby/indent/ruby.vim:189
Called 201 times
Total time:  16.409863
 Self time:   0.055199

And here it is when it calls the vim9 function:

FUNCTION  GetRubyIndent()
    Defined: ~/.vim/bundle/ruby/indent/ruby.vim:53
Called 201 times
Total time:  11.736142
 Self time:   0.003108

It's nothing too dramatic, but it's not nothing, I guess.