Automate releases and standardize build step infrastructure

[choldgraf]

This repository recommends using GitHub releases to distribute the source code for plugins for others to re-use. E.g. see how we're using releases here:

myst-plugins/myst.yml

Lines 14 to 26 in a4df31e

	plugins:
	- https://github.com/jupyter-book/myst-plugins/releases/download/Admonitions/experiment-admonition.mjs
	- https://github.com/jupyter-book/myst-plugins/releases/download/exercise-and-solution-pdf/exercise-admonition-pdf.mjs
	- https://github.com/jupyter-book/myst-plugins/releases/download/Admonitions/intermezzo-admonition.mjs
	- https://github.com/jupyter-book/myst-plugins/releases/download/Admonitions/example-admonition.mjs
	- plugins/picsum/picsum.mjs
	- plugins/markup/markup.mjs
	# - plugins/iframe-to-thumbnail-pdf/iframe-to-thumbnail-pdf.mjs
	- https://github.com/jupyter-book/myst-plugins/releases/download/iframe-to-qr-pdf/iframe-to-qr-pdf.mjs
	- https://github.com/jupyter-book/myst-plugins/releases/download/updated-date-frontmatter/update-date-frontmatter.mjs
	- plugins/github-issue-link/src/index.mjs
	- plugins/github-handle-links/src/index.mjs
	- plugins/demo/src/index.mjs

But AFAICT, the releases are done manually, rather than through some kind of automation.

Additionally, some plugins require a 'build' step before they create a usable JS bundle, e.g.:

myst-plugins/plugins/update-date-frontmatter/build.js

Line 4 in a4df31e

We should:

1. Automate creating a release for any plugin that has changed since the last release.
2. Define a standard build step (maybe it is just "create a file called build.js and if it exists, the release workflow will run that and look for a file in a dist/ folder and publish whatever it finds there in a release for that plugin)