Fix #827: Implement java.net.URI
Merge pull request #855 from gzm0/uri

Fix #827: Implement java.net.URI
Intern scalajsenv.js via a source generator.

We fully remove the tool's dependency on I/O and Java-style resources
by generating a .scala source file that contains the whole
scalajsenv.js as a literal string.
Merge pull request #861 from gzm0/internalize-env

Intern scalajsenv.js via a source generator.