Allow to selectively enable/disable features of this extension

We see several demands from users to selectively deactivate features of this extension. This is either because of performance reasons (long running queries in the background) or simply because features are not used and hence dependencies shouldn't be required (e.g. buildifier).

Features that qualify for deactivation:
- Target Tree in the explorer view
- CodeLens
- Bazel Commands requiring bazel (build/test/queries)
- Syntax Highlighting (? is this really something that you would want to turn off)
- Language Server
- Buildifier Linting

I assume there are some pitfalls to avoid here:
- Deactivating bazel commands will also render almost any other functionality useless
- We already have some settings that implicitly deactivate some features when the executable path is empty (language server). We might need to introduce a breaking change in order to provide a consistent user interface for configuration
- A lot of the setup code for the features is done during activation of the extension. It probably requires quite some refactoring to be able to deactivate features without requiring a reload of the complete IDE.

For reference, I saw demands of this here (and some of these could be closed if this issue is solved):
- https://github.com/bazel-contrib/vscode-bazel/pull/451
- https://github.com/bazel-contrib/vscode-bazel/issues/227
- https://github.com/bazel-contrib/vscode-bazel/issues/193
- https://github.com/bazel-contrib/vscode-bazel/issues/402
- https://github.com/bazel-contrib/vscode-bazel/issues/403
- https://github.com/bazel-contrib/vscode-bazel/issues/449
- https://github.com/bazel-contrib/vscode-bazel/issues/595

Start by mapping the extension activation and setup code for the Target Tree, CodeLens, Bazel commands, syntax highlighting, language server, and Buildifier features. Define a consistent configuration approach for selectively disabling each feature, including dependency requirements and whether changes require an IDE reload; done means each listed feature can be independently disabled without breaking the remaining extension.

Written by the indexing model from the issue text.