#6 Add studies.update_chapter_tags

[nstillman-te]

Part of #6.

Adds client.studies.update_chapter_tags(study_id, chapter_id, pgn) for the Lichess study chapter tags endpoint (POST, form-encoded body, 204 No Content). Session handles 204 by returning None. Requests-mock test for path and form body.

Checklist when adding a new endpoint

• Added new endpoint to the README.rst
• Ensured that my endpoint name does not repeat the name of the client. Wrong: client.users.get_user(), Correct: client.users.get()
• Typed the returned JSON using TypedDicts in berserk/types/ — N/A (endpoint returns 204 No Content, no response body)
• Written tests for GET endpoints not requiring authentification — N/A (POST endpoint; added requests-mock test for request path and body)
• Added the endpoint and your name to CHANGELOG.rst in the To be released section