Fix messed up branch state before v3 release

Problem

Because we branched off v3 and later made that the default branch, some PRs sneaked into master when they shouldn’t have. We also did a release off master and not v3 so they got further out of sync.

Solution

In order to release everything and get the branches in a good state again I suggest we do the following:

1. https://github.com/xtermjs/xterm.js/pull/1185 Merge master into v3, this will be a small easy to understand PR that’s easy to sign off on and that we know is in a good state as it’s based on v3.
2. https://github.com/xtermjs/xterm.js/pull/1186 Merge v3 into master, this diff will contain everything in the release so it can’t really be reviewed
3. Mark master as the default branch
4. Update package.json version in master to 3.0.0
5. Release 3.0.0 from master (@parisk to sign commit)
6. Branch off release/3.0 from `master
7. Tag HEAD of release/3.0 as 3.0.0
8. Move all PRs to point at master branch
9. Publish release in Github

If we need to release 3.0.1+ then we can add the commits to master, cherry pick them to release/3.0 and release again. We can then continue this process for 3.1 with release/3.1 and so on to avoid these sorts of problems in the future; the master branch will always be the latest and we branch off stabilization branches for release.

[Tyriar]

And it’s out!

[exsilium]

Congratulations on this major milestone! 🥇 🏆🍾🎉