docs(editor-languages): document register options and formatAs

[bajrangCoder]

• Documents the options argument of editorLanguages.register (aliases, filenameMatchers, formatAs).
• Adds a Formatting section explaining formatAs, added in feat(modes): let plugin modes reuse another mode's formatter Acode#2976.

[greptile-apps]

Confidence Score: 5/5

[Low impact] The documentation change appears safe to merge; no actionable issues were found.

Summary

Documents the optional options argument for editorLanguages.register.

• The editor-language guide explains registration options and formatter reuse.

No actionable issues were established.

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