Add Vim9 Script

[DanBradbury]

Adds vim9 script as a new language.

Description

Since the syntax is quite different from legacy vimscript I've added this as a new language and added a heuristic since all vim9 script files must start with vim9script on the first line. This allows for clear detection of vim and vim9script within repos

While there are not 100+ repos using vim9script I believe a large reason for the lack of adoption is the lack of syntax highlighting support that makes some of our efforts to use modern vimscript harder for new plugin writers.

97.40%  40549      Vim9 Script
1.25%   521        Vim Script
0.77%   320        Dockerfile
0.58%   242        Shell

Dockerfile:
  .devcontainer/Dockerfile

Vim Script:
  .devcontainer/init.vim
  .devcontainer/vimrc

Shell:
  .devcontainer/setup.sh
  move.sh
  run-tests.sh

Vim9 Script:
  autoload/copilot_chat.vim
  autoload/copilot_chat/api.vim
  autoload/copilot_chat/auth.vim
  autoload/copilot_chat/buffer.vim
  autoload/copilot_chat/config.vim
  autoload/copilot_chat/debug.vim
  autoload/copilot_chat/history.vim
  autoload/copilot_chat/models.vim
  ftplugin/copilot_chat.vim
  plugin/copilot_chat.vim
  syntax/copilot_chat.vim

Checklist:

• I am adding a new language.
  • The extension of the new language is used in hundreds of repositories on GitHub.com.
    • Search results for each extension:
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.vim+vim9script
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/DanBradbury/copilot-chat.vim/blob/main/autoload/copilot_chat.vim
    • Sample license(s): MIT
  • I have included a syntax highlighting grammar: [URL to grammar repo]
  • I have added a color
    • Hex value: #007030
    • Rationale: Close to vimscript's selected color but different enough to clearly see the difference.. also the color for my alma mater
  • I have updated the heuristics to distinguish my language from others using the same extension.

[lildude]

The test failures indicate you've not followed the instructions in the CONTRIBUTING.md file for adding a language and grammar. Please do so.

While there are not 100+ repos using vim9script I believe a large reason for the lack of adoption is the lack of syntax highlighting support that makes some of our efforts to use modern vimscript harder for new plugin writers.

I can't help but feel this PR is a very premature. You're barely over 100 .vim files which is a long way from our requirements (see the issue linked in the CONTRIBUTING.md file) and there are only 2 .vim9 files. All extensions must meet our usage requirements before a PR can be merged.

[DanBradbury]

@lildude RE popularity; I'm wondering if the search feature isn't working correctly since the results don't show a single one of my own plugins or files that are written in vim9 script. I also don't see many plugins that are a part of the awesome-vim9 list (77 repos on that list alone which doesnt match the Repo count shown either).

Very possible I messed up the query last night when opening the PR

---

To add additional context for my thinking on search being broken.

I have a repo https://github.com/DanBradbury/copilot-chat.vim that is not a fork and includes vim9 script files in the default branch.. however with the default search none of my files show up in the query.

If I add an additional param to include the repo itself then I get 11 file results (see https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.vim+vim9script+repo%3ADanBradbury%2Fcopilot-chat.vim+)

When doing a cursory look on the files pulled up I'm not seeing a lot of repos from awesome-vim9 that I know are valid vim9 script plugins that are not forks. The reason I'm so familiar with these repos is because of my work on a vim9 linter where I test against these repos (https://github.com/DanBradbury/vinter/blob/main/vinter_report.md)

[DanBradbury]

@lildude - I tried my best to address your comments + include another advanced.vim in samples. If there are more files required I'm happy to continue adding more vim9 script samples (can breakdown by features if that helps)