Add a formatter guide under "Language Extensions"

[octref]

• Talk about document formatter, selection formatter, format on paste/save
• Point out 2 approaches: use API vs LSP
• Stress on dynamic registration/disablement on setting

Provide a sample too. One I can think of:

• Doc formatter: Remove trailing empty line
• Range formatter: Remove trailing whitespace characters

[gregvanl]

@octref Do we still need this? Looks like it is partially covered in https://code.visualstudio.com/api/language-extensions/programmatic-language-features#format-source-code-in-an-editor

[gregvanl]

Closing old work item issue without upvotes