feat(setContext): hoist setContext into a shared module

[justinmk3]

Problem

The vscode 'setContext' command:

• is not type-checked
• usages cannot be found programmatically (i.e. via "find references")
• keys are not centralized, so it's not clear which keys we are setting

Solution

• Introduce a setContext module. Define keys as a type.
• Define a custom lint rule which prevents direct use of the vscode command.

License

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[justinmk3]

/runintegrationtests