Add %PUBLIC_URL% support

[drwpow]

From #464, adds the ability to set buildOptions.baseUrl in the Snowpack config. This replaces %PUBLIC_URL% with that at build time, and / at dev time.

Given baseOptions: { baseUrl: '/static/' }…

Dev
Note: all URLs start with /

Build
_Note: all URLs start with /static/ from our config! 🎉 _

Source HTML
Note: all URLs start with %PUBLIC_URL%

[FredKSchott]

LGTM!

[danprince]

@drwpow What's the plan for getting the baseUrl into the rewritten imports statements (as in #476)? Want me to update that branch with the new baseUrl?

[drwpow]

Yes I believe that makes sense! If my thinking is correct, I think the path for web modules should be something like:

[baseUrl] / [webModulesDir] / some-dep.js

Keeping in mind that both baseUrl and the webModulesDir (whatever that ends up being called) can both be multiple folders, not just one. I think this would work for most setups.