Add Pylance to languageServer setting enum

[joyceerhl]

After a user installs Pylance, Pylance updates the python.languageServer setting to 'Pylance', which appears to not be a recognized option for that setting. The user sees this:

Just had a customer report this here: https://github.com/microsoft/vscode-python/issues/12751#issuecomment-654875902

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR).
• Title summarizes what is changing.
• Has a news entry file (remember to thank yourself!).
• Appropriate comments and documentation strings in the code.
• Has sufficient logging.
• Has telemetry for enhancements.
• Unit tests & system/integration tests are added/updated.
• Test plan is updated as appropriate.
• package-lock.json has been regenerated by running npm install (if dependencies have changed).
• The wiki is updated with any design decisions/details.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities (and 0 Security Hotspots to review)
0 Code Smells

No Coverage information
0.0% Duplication

[MikhailArkhipov]

Please don't merge just yet. This was intentional.

[MikhailArkhipov]

#13123 takes over this