Provide a npm module that simplify testing of language features on single virtual file

[octref]

Extract those helpers from the html/css Language Services and put them into a npm module.

With this we can expand our Language Server tutorial with a section "testing" using this module.
Hopefully this module could also help other Language Server authors.

Todo:

• Extract the helpers into a npm module
• Migrate html/css LS's test to use it
• Migrate LSP samples to use it for testing
• Update LS authoring doc with testing section

[vscodebot]

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