Enable on-type formatting from language server

[Jake Bailey (jakebailey)]

For #2690.

Similar to #2612. Note that :-based formatting isn't implemented in language server yet (microsoft/python-language-server#165), but it hasn't been enabled in the extension for anyone for a while now either (fixed in #2714/#2724).

Since line formatting is relatively new in language server, this likely shouldn't make it out into a release until the language server downloaded by default is new enough to include that functionality. I haven't been monitoring that side of things, so feel free to hold off on merging this until the right time.

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Has a news entry file (remember to thank yourself!)
• Unit tests & system/integration tests are added/updated
• Any new/changed dependencies in package.json are pinned (e.g. "1.2.3", not "^1.2.3" for the specified version)
• package-lock.json has been regenerated by running npm install (if dependencies have changed)

[Don Jayamanne (DonJayamanne)]

Rejecting, to ensure this doesn't get merged for this release

[Jake Bailey (jakebailey)]

Turns out I forgot to add the news file before I made the PR (so it's good that this was waiting and not merged). It's added now.

[Jake Bailey (jakebailey)]

For reference, a block formatting port was merged in PLS in microsoft/python-language-server#217.