Tool window to select default `--config` flag value from values defined in `.bazelrc`

This is a feature request / idea.

As documented [here](https://bazel.build/extending/config#using-build-setting-aliases), the `.bazelrc` file allows defining aliases for collections of command lines arguments to pass to the Bazel commands, like this:
```
:
```

I think it would be valuable to offer the user an easy way to switch between configurations for each command (or maybe also globally?) in a UI element in the editor. I got the idea from looking at #292.

@jfirebaugh what do you think?

Start by reviewing the `.bazelrc` alias syntax described in the issue and the UI work referenced by #292. Determine how command-specific or global configuration selection should behave in the VS Code extension. Done means the editor offers a defined way to choose available `.bazelrc` configurations and applies the selected value to the relevant `--config` flag.

Written by the indexing model from the issue text.