Add WXML (WeiXin Markup Language)

[OverflowCat]

Description

WXML (WeiXin Markup Language) is a markup language mainly used in Weixin Mini Programs.

There are WXSS (WeiXin Style Sheets) and WXS (WeiXin Script, usually not in a separate file) but I cannot find corresponding grammar files. See #7173

Also, I am not sure if the grammar files is working. Can we ignore some files in the submodule so that we do not need to fork and delete some json files?

Checklist:

• I am adding a new language.

  • The new extension is used in hundreds of repositories on GitHub.com

    • Search results for each extension:
      • NOT is:fork path:*.wxml /<(view|text|template|import|block)>/ yields 63k files

  • I have included a real-world usage sample for all extensions added in this PR:

    • Sample source(s):
      • https://github.com/youzan/vant-weapp/blob/dev/lib/calendar/calendar.wxml (17.9k stars)
      • https://github.com/iamxjb/winxin-app-watch-life.net/blob/master/pages/index/index.wxml (2.8k stars)
    • Sample license(s): MIT

  • I have included a syntax highlighting grammar: https://github.com/OverflowCat/wxml-tmlanguage

  • I have added a color

    • Hex value: #07c160
    • Rationale: Button color suggested in Weixin Mini Program Design Guidelines (only in Chinese version)

  • I have updated the heuristics to distinguish my language from others using the same extension.