feat: better progress ux

[mhuisi]

This PR improves two things about the way we report progress:

• Instead of attempting to badly approximate an accurate progress bar that gets stuck whenever the output of a command doesn't change, we now use VS Code's "infinite" progress bars everywhere that clearly show that an operation is actively running in the background.
• Every progress bar is prefixed with a context to make it clear which operation caused the operation. For example, when creating a new mathlib project, all external commands executed while creating the project are prefixed with "[Create Mathlib Project]".

Closes #457. IMO, our dialog UX still isn't perfect, but there doesn't seem to be a good way to improve these things short of implementing our own webviews for every command.