Contributes to #1576

What did you do?

moved postinstall to its own js script

Why did you do it?

fix npm 10+

How have you tested it?

This time test both installing via a tgz file AND installing from a local registry.
Test steps:

• Confirm that installing 3.0.18 fails with npm 10 when installing (see reproduction steps in Cannot install carbon-vue on fresh Vue 3 project #1576)
• Confirmed that the same 3.0.18 also fails when installing with npm 10 from a local npm registry (see compose in local-dev and the updated README for how to test this.)
• Changed postinstall to a script that detects prod/dev install.
• Built new version and posted this version to local npm registry
• Confirmed that the new 3.0.18+ installs properly with npm 10 from a local npm registry
• Confirmed that the dev install (yarn install) still installs the husky hooks & storybook custom controls

Were docs updated if needed?

• Yes