Extract `options.localServePath` option.

We’re currently computing a “local servePath”, or the pathname of the servePath option with the leading slash removed, if it exists.

This “local servePath” enables us to generate paths to assets using a fully-qualified path (something like “http://localhost:2415/assets/file-d41d8cd98f00b204e9800998ecf8427e.js”) but then serve them at that local servePath (“/assets/file-d41d8cd98f00b204e9800998ecf8427e.js” in this example).