Keep track of a text document's `language_id`

[alcarney]

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

This ensures that pygls remembers the language_id of text documents opened by the client.

Closes #212

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
• [/] Standalone docs have been updated accordingly
• [/] CONTRIBUTORS.md was updated, as appropriate
• [x] Changelog has been updated, as needed (see CHANGELOG.md)

[tombh]

What a productive day!