vim: support new ":def" functions of vim9

[lacygoill]

This PR tries to add support for the new :def functions introduced in Vim9. See issue #2589 for more information.

I briefly tested the patch, and it seems to work. That is now ctags(1) correctly generates tags for :def functions.

But I don't know if it's really correct; I don't know C.

If it is correct, I think I could also add support for constants defined with the :const keyword.

[coveralls]

Coverage remained the same at 86.777% when pulling 1a44f70 on lacygoill:vim9-def-functions into b4574f0 on universal-ctags:master.

[codecov]

Codecov Report

Merging #2590 into master will decrease coverage by 0.00%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master    #2590      +/-   ##
==========================================
- Coverage   86.67%   86.67%   -0.01%     
==========================================
  Files         183      183              
  Lines       38608    38608              
==========================================
- Hits        33464    33462       -2     
- Misses       5144     5146       +2     

Impacted Files	Coverage Δ
parsers/cxx/cxx_parser.c	85.42% <60.00%> (ø)
parsers/vim.c	99.59% <100.00%> (ø)
parsers/cxx/cxx_parser_variable.c	90.56% <0.00%> (-0.63%)	⬇️

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[masatake]

Could you change the first line of the commit log to:

vim: support new ":def" functions of vim9

because we expect the first word of the commit log is names of parsers or names of subsystems.

We need a test case. Units/parser-vim.r is the directory for the vim parser. I need a test case for :def here.
See https://docs.ctags.io/en/latest/units.html . You can tests the vim parser with:

$ make units LANGUAGES=Vim

[masatake]

@lacygoill, please look at 1a44f70 .

A test case consists of two files: an input file and an expected output file.
You prepared only an input file. The expected output is so obvious. So I added it.

When adding code for :const, please include both files for new code to your commit.

[masatake]

Thank you for your contribution. I'm waiting for the patch for :const:-)

[lacygoill]

Thank you very much for merging the PR.