feat: change tracking and revert

[azorng]

If after submitting a prompt goose has made changes to our files, we can review the file changes (diff) and/or revert them.

default mappings:

      diff_changes = '<leader>gd',           -- Display a vertical split diff of changes since the last goose prompt
      revert_all = '<leader>gra',            -- Revert all file changes since the last goose prompt
      revert_this = '<leader>grt',           -- Revert current file changes since the last goose prompt

closes #39