Add `@angular-builders/custom-esbuild:unit-test` builder

[th0r]

Is your feature request related to a problem? Please describe.
Currently there is no way to run unit tests having custom esbuild plugins as @angular-builders/custom-esbuild has only 2 builders: application and dev-server.

Describe the solution you'd like
There is an official (but still experimental) @angular/build:unit-test builder which uses esbuild to build an app and Vitest as a test runner.

Can we add a corresponding @angular-builders/custom-esbuild:unit-test builder that will wrap it and allow to provide an extra esbuild plugins just like @angular-builders/custom-esbuild:application does?

Describe alternatives you've considered
I couldn't find any other alternative.

[th0r]

@just-jeb could you share your thoughts about it please? Do you have plans to implement it or you have some concerns?

I'm asking because this missing unit test builder blocks us from moving to esbuild-based build and if you don't have plans to add it to custom-esbuild then I guess we'll have to write something of our own.

[just-jeb]

Shouldn't be a problem. The main concern is its support for builder extensions (similarly to application builder) and looks like it's supported.
So technically I don't see a problem, it's just a matter of finding time for this. Would you like to give it a go and create a PR?

[th0r]

@just-jeb yes, I'll try to implement it, thanks.

[just-jeb]

Thank you! Lmk if you need any guidance/help 🙏.

[th0r]

@just-jeb I've created a PR: #1935

Could you take a look please?

[th0r]

@just-jeb ping

[just-jeb]

Great job 🙌 Reviewed and commented 🙏