Experiments: add extension Commands for gc and exp --queue/--run-all

[rogermparent]

A subset of #239

These commands are alike in that they can be run with no provided Experiment.
To be more explicit, the added commands are:

• exp run

  • --queue Add Commands for exp queue, exp run --run-all, and exp gc #251
  • --run-all Experiments Commands #2: Add "Run All Queued Experiments" Command #268

• exp gc Experiment Commands #3: Add command for exp gc with selectable flags #269

  • --workspace
  • --all-branches
  • --all-tags
  • --all-commits

We'll probably want a VSCode command for each one so users can select them from the command palette.

We could handle gc's multiple flags with a menu like a quickpick, but it may be best to have individual commands either way for easy command palette access, then maybe we can make an extra command that invokes a quickpick menu so that all can be accessed from a single keybind.

[rogermparent]

#251, #268, and #269 together should fix this issue