Memoize producers and originators for improved hot reload speeds

[shah]

For hot reloading of *.md and *.html, the most commonly edited files, hot reloading should be much faster and not require rebuilding the entire site just for a single file refresh.

• memoize *.md source and producer
• memoize *.html source and producer
• edited files should auto-refresh the browser
• added files should auto-restart the server
• removed files should auto-restart the server

[shah]

For some reason, when editing a file in WSL on Windows 11, the following duplicate events behavior is exhibited:

We should figure out if that's an issue in Deno or RF.