Move commands to constants and clean up circular imports

Move all existing command strings to commands.ts
Move some existing strings used in multiple places to constants.ts
Create interfaces for existing classes to clean up circular import references. There's still a few left but this PR is large enough as is.
Remove some more commented out code.

This PR when merged should fail the deploy step. We are not incrementing the version here, just making future building easier.

launchdarkly / ld-vscode