Changelog: Add changelog entries for Pulsar 1.121.0

[DeeDeeG]

Adds Changelog entries for a bunch of PRs, specifically the ones which we have landed in core repo since Pulsar Regular release 1.120.0 was tagged.

Note: pulsar-edit/ppm#134 was included in error in the 1.120.0 changelog, despite not being present in the ppm bump to core repo that time around. I have rectified this oversight by moving the entry to this release's notes. So, there is the rare change to a prior release's CHANGELOG.md notes in this PR. It serves the purpose of accuracy in documentation, even if said accuracy is late in arriving.

[DeeDeeG]

Thanks for the review! I may let @savetheclocktower take a look as well, in case they don't want quite so many lines from the Tree-sitter PR in the Changelog, or if they have views on how these entries are presented in the Changelog (though I simply copy-pasted the bullet points from the "Release Notes" section of the PR, so it seems reasonable enough and a safe move.)

I won't be finishing the Regular release this evening either way, so in that sense I figure there is no rush.

It is okay for anyone to merge this, given the approval, especially if an hour or so have gone by with no changes requested. (I personally will return to it tomorrow, need to get some sleep for now.)

[savetheclocktower]

Looks good to me.