Allow support for subset YAML languages
e.g. yaml-textmate, yaml-tmlanguage
RedCMD/TmLanguage-Syntax-Highlighter#19

This would allow extension authors to create custom highlighting for their subset yaml language
as VSCode doesn't allow overriding the default syntax highlighter for a particular file glob pattern without making an entirely new language
custom highlighting:

both https://marketplace.visualstudio.com/items?itemName=pedro-w.tmlanguage and (in the future) https://marketplace.visualstudio.com/items?itemName=RedCMD.tmlanguage-syntax-highlighter would like to enable this YAML extension on their subset yaml languages

a (temporary) specific solution for just us ^ PR: #1093

otherwise a generic solution for everyone can be found here:

• [json] Allow extensions to enable the builtin json-language-features extension on their own personal json languageId microsoft/vscode#198025
• Contribute to json language server with a custom language. microsoft/vscode#198583
• Contribute to html language server with a custom language. microsoft/vscode#146730
• Contribute to html language server with a custom language. microsoft/vscode#146731

"yamlLanguageParticipants": [
	{
		"languageId": "foobar"
	}
]