Only update desired key in `python.analysis.diagnosticSeverityOverrides`

[jrwagz]

This should ensure that only the config key we care about is updated, and not every other config that the user has set.

As mentioned in https://github.com/joedevivo/vscode-circuitpython/issues/105

I am NOT a TypeScript developer, but I am a Python developer. So please review this carefully, as I am not sure how to properly test it, nor do I write TypeScript professionally.

[evilhamsterman]

Should probably also just set "reportShadowedImports": "none" by default too as it seems to be something everyone sets because code.py conflicts with Python's code library

[joedevivo]

Merged #123, which I think covers this as well

[evilhamsterman]

Yes it should @joedevivo