Remove MobX-State-Tree flag for non-demo projects

[Jpoliachik]

This PR adds support for removing MobX-State-Tree code from a non-demo project. Retargeted to v10 branch.

Changes:

• // @mst inline comments, similar to how demo code removal works
• Refactored demo.ts removal logic into more generic markup.ts to be shared with @mst comment types too, and make it easy to add additional other types in the future too.
• Added new replace-next-line markup command type, to help swap out lines of code easily without hot patching in scripts (keeps code replacements inline next to other code)

TODOs:

• README update

(I'll also add some additional screenshots & videos for testing proof soon)

[frankcalise]

@Jpoliachik can you target this against v10 as the base branch?

[Jpoliachik]

@frankcalise sure thing, I'll work on this early next week 👍

[Jpoliachik]

This is ready for additional review.

As far as CLI documentation, I added the --mst flag option to the list, and added a blurb under the MST page. I didn't think much else was necessary, since this flag should be straightforward for users, but i'm open to opinions if we think other docs are needed!

[infinitered-circleci]

:tada: This PR is included in version 10.0.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: