cleanup: restructure `context.ts` into subdir

[shouples]

Summary of Changes

Closes #521.

Creates a new context/ subdir and splits the existing context.ts into context/extension.ts (for general ExtensionContext use) and context/values.ts (for all our context value management).

This is in preparation for an upcoming branch dealing with observability context to address https://github.com/confluentinc/vscode/issues/449

Pull request checklist

Please check if your PR fulfills the following (if applicable):

Tests

• [ ] Added new
• [x] Updated existing
• [ ] Deleted existing

Other

• [ ] All new disposables (event listeners, views, channels, etc.) collected as for eventual cleanup?
• [ ] Does anything in this PR need to be mentioned in the user-facing CHANGELOG or README?
• [ ] Have you validated this change locally by packaging and installing the extension .vsix file?

  gulp clicktest