Respect client's preferred encoding when possible

[nthykier]

Description (e.g. "Related to ...", etc.)

Related to #445

This change would make pygls accept the client's preferred encoding rather than preferring UTF-16 blindly. For editors that prefer UTF-32 this is advantageous for pygls since the position encoding/decoding becomes simpler.

Code review checklist (for code reviewer to complete)

• [X] Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• [X] Title summarizes what is changing
• [X] Commit messages are meaningful (see this for details)
• [X] Tests have been included and/or updated, as appropriate
• [X] Docstrings have been included and/or updated, as appropriate
• [X] Standalone docs have been updated accordingly

[tombh]

Thank you. I'll merge it now so it's available on the main branch. I won't cut a release immediately. But if you want a release soonder, please feel free to let us know.

[nthykier]

Thanks for the merge. No urgent need for this released. :)