This commits sets the JS target for all command line tools to
NodeJS v12. ESbuild will automatically downlevel the ES2020 features
we currently use to make them compatible with NodeJS v12 <-> ES2019.
ES2020 is the prodmode output, but we still support Node v12 so
there needs to be some downleveling for now.
Note: This is a separate commit because initially the target was
set to Node v14 to match up with the prodmode Bazel output.
PR Close#43431