Feature Request: Custom Builder for Karma Tests Using esbuild

[ckorherr]

Is your feature request related to a problem? Please describe.
We are transitioning to the esbuild builder in our application. The only remaining component that still relies on Webpack is our Karma unit tests. We use custom-webpack for these tests due to the need to modify the build process.

Describe the solution you'd like
We would like to request a custom builder for running Karma tests using esbuild. This would allow us to maintain a consistent build process by using a single type of builder.

Describe alternatives you've considered
We have considered continuing to use both Webpack and esbuild, but this approach complicates our build process and maintenance.

Additional context
Some of our unit tests require loading the Monaco Editor, which is currently achieved using custom builders. A custom builder for Karma tests with esbuild would simplify this setup.