long term cache workspace and node module files in development

[thescientist13]

Type of Change

• New Feature Request
• Documentation / Website
• Improvement / Suggestion
• Bug
• Other (please clarify below)

Summary

During development, files like node_modules rarely changing, at least while the dev server is running. Source code files only only change when they, well, change.

This means we should be able to do some really easy and intelligent caching during the lifetime of the development session so repeated requests to files on disk, like node_modules, don't have to initiate a filesystem read. (See #628 )

Details

I think the technique for this is using an E-Tag, which stores a hash of the content as a header and uses it for comparing at request time to see if the header still matches and if not, just return a 304 Not Modified Header. For node_modules we can assume this after first fetch. For user code, only the actual JS, CSS, etc file that changed should actually be refetched.

Additionally, we only need to build importMap once as well, so we should cache that too and not do it on intercept of an HTML page.