tsconfig: enable incremental

[pjonsson]

What this PR does

This makes tsc keep a file in
the output directory that tracks
what has been compiled previously,
and avoids recompiling that unless
it has changed.

Running yarn gulp build two times
in a row will have the first run
spawn a node that consumes ~40 CPU
seconds, and the second run the node
process consumes 10 CPU seconds.

Test me

Run yarn build-for-node two times in a row, and notice that the second time is almost instant.

Checklist

• There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• I've updated relevant documentation in doc/.
• I've updated CHANGES.md with what I changed.
• I've provided instructions in the PR description on how to test this PR.