This enhances RollupCompiler to explicitly look for .mjs and .ts

variants of the Rollup configuration files, and to use the correct

configuration arguments for each file type. This will give projects a

lot more flexibility in how they set up and even split up their Rollup

bundling.
It also checks for other common JavaScript configuration files

(babel.config.json and tsconfig.json) to try to calculate the top of

the tree.

Temporarily changed Review Board to use both .mjs and .ts files,

and verified that the right build configuration was used for each.