Improves build script performance by:
* using built-ins wherever possible (e.g. string splitting)
* eliminating redundant code
* using absolute (to $KEYMAN_ROOT) rather than relative paths to avoid realpath
* removing unnecessary `npm run` calls
BEFORE | AFTER
-------------------------------|----------------------
time ./core/build.sh --help |
real 0m2.116s | real 0m0.874s
user 0m0.578s | user 0m0.198s
sys 0m0.984s | sys 0m0.289s
-------------------------------|----------------------
time ./web/build.sh --help |
real 0m3.523s | real 0m0.757s
user 0m1.166s | user 0m0.320s
sys 0m2.273s | sys 0m0.455s
-------------------------------|----------------------
time ./web/build.sh -d |
real 1m34.750s | real 0m59.974s
user 0m9.721s | user 0m6.284s
sys 0m19.652s | sys 0m13.202s
@keymanapp-test-bot skip
