upgrade esbuild

[cloverich]

• upgrades esbuild to latest version, as part of core dependencies review
• incorporate lint into build step, and prefer yarn run build to calling ./build.sh directly
• update README and some documentation (comments)

Had to refactor the prior esbuild callbacks to plugins, which was easy enough once I figured out what to look for. The build itself is a bit slower, but still acceptable. Definitely a few improvements left on the table:

• Renderer code should use a live or hot reload server, instead of re-starting the electron process
• Type checking could be more thoroughly integrated (fail the bundle), and faster (incremental? stateful process?)

Overall probably worth looking into third party libraries that have likely sorted this out well; now that the project is on the latest esbuild version that should be easier.

Likely the last batch of work I'll do as part of #118