Review Board's extension packaging support now uses the new static media

extension packaging classes introduced in Djblets 5. These gives us the

ability to hook into the build process to generate configuration files

for building modern JavaScript/TypeScript codebases.
When building an extension, the following files are now created if they

don't already exist:

babel.config.json
rollup.config.mjs
tsconfig.json

These all provide basic defaults suitable for the extension, and are set

to inherit the versions of these files packaged with Review Board.
On top of this, the JavaScript/TypeScript codebases from the installed

Djblets and Review Board packages are included as NPM workspaces, offering

@beanbag/djblets and @beanbag/reviewboard ESM module imports. This is

primarily useful for TypeScript definitions.
Currently, the configuration and workspace links are only generated when

first building an extension. In the future, we'll want to generate them

when creating an extension in the first place.

Successfully built rbintegrations against this, and verified that the

files and workspaces were present.