feat(common): establish builder.typescript.inc.sh and friends

> 2. From a maintenance view, the `builder_` prefix would make me assume this function is defined in `builder.inc.sh` with all the other `builder_` functions.

I didn't want to put this into the main builder script, as it is specific to Typescript (and perhaps specific even to Developer use of Typescript; not sure it would transplant to /web for example). Ideally, we may have a set of builder sub-scripts, e.g. builder.typescript.inc.sh, builder.meson.inc.sh, etc, which we can draw from, and I would like to consider that for 19.0 (perhaps low pri though). shellHelperFunctions.sh is a bit of a random grab bag of functions that need refactoring and renaming, and eventually it should go away.

_Originally posted by @mcdurdin in https://github.com/keymanapp/keyman/pull/12736#discussion_r1862818255_