Switch build system from ncc to esbuild

[mcous]

Overview

This PR changes the build system from the Webpack-based ncc to ESBuild for speed and security. ESBuild is much faster than ncc and has no additional dependencies, unlike ncc which is (relatively) slow and pull in many dependencies.

Notable changes

• ESBuild appears to be successfully bundling the semver dependency, so it's no longer required to commit node_modules/semver to revision control
• Weird coverage-related failures in Windows CI have beeen resolved
• src has been added to the npm package so compiled JS can link back to the TS source

Future work

After this build change is in, the following items can be worked on

• Upgrade TypeScript and ESLint to recent versions
• Pull in Vitest
• Simplify module internals
• (Maybe) Move asset generation to a CI workflow