This introduces a new compiler for Pipeline that interfaces with

rollup.js to build JavaScript. Rollup can itself interface with Babel,

TypeScript, etc., and can handle resolving ES6 modules. This allows

consumers (including us) to begin modernizing JavaScript codebases.
The Rollup Pipeline compiler defaults to handling any index.js,

index.es6.js, or index.ts files included in the Pipeline bundle. It

will then compile the file and include its contents in the resulting

Pipeline bundle.
This allows projects to iteratively move to Rollup-bundled JavaScript

with module support, while still keeping legacy JavaScript in the same

bundle. Over time, projects can phase out all but the index file,

allowing Rollup to handle all the actual JavaScript compilation and

allowing Pipeline to handle versioning, resolving, and packaging the

resulting file.
Upcoming changes will begin to introduce usage of Rollup within the

Djblets codebase.

Tested this along with the rest of the modern JavaScript work.
I've verified that Rollup correctly executes within the right

environment for the source files being built, and respects the

appropriate configuration files.
Verified that outdated file checks take into account any files

included in the sourcemaps, and correctly triggers a rebuild only

when there's a change.