Fix sourcemap with path prefix when using esbuild with publicPath

[vzaramel]

Esbuild puts the publicPath as a prefix in the sourceMappingURL and because I am using editor/editor to generate the script tag sprockets-rails generating the wrong path and removing the sourceMappingURL from the compiled file.

 <%= javascript_include_tag 'editor/editor', defer: true, integrity: true, crossorigin: "anonymous" %>

// build.js
esbuild
  .build({
    bundle: true,
    sourcemap: true,
    format: 'esm',
    outdir: path.join(process.cwd(), '../../app/assets/builds/editor'),
    publicPath: './editor',
    entryPoints: ['packs/editor.js'],
    ...

//  app/assets/builds/editor/editor.js
.
.
.
//# sourceMappingURL=editor/editor.js.map

This PR fixes it by only running the special case path when sourceMappingURL does not have a directory prefix.

[lcmen]

Hey @vzaramel, thanks for this PR!
Unfortunatelly, this code doesn't work for our case (publicPath set to /assets so static files can be correctly resolved when imported in JS files) as our sourcefile was application but sourcemap was /assets/sourcemap.js.map.

I took the inspiration from your code and fixed it (temporarily) like this:

Sprockets::Rails::SourcemappingUrlProcessor.class_eval do
  def self.combine_sourcemap_logical_path(sourcefile:, sourcemap:)
    if (parts = sourcefile.split('/')).many?
      parts[0..-2].append(sourcemap).join('/')
    elsif (parts = sourcemap.split('/')).many?
      parts.last
    else
      sourcemap
    end
  end
end