feat(bundler): add a bundler layer

[johnjbarton]

The input config supplies 'patterns' which FileList expands into Files.
These files are preprocessed and then 'file-list-modified' is fired.
The event offers a pair of file lists, {included, served}.
The bundle layer maps this pair to a new pair where fewer files are included.
By default the bundler copies the inputs to outputs.

The bundled files are stored on currentWebFiles which replaces
the FileList.files in the middleware/karma.

Fixes #3633

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[jginsburgn]

Maybe we can display a warning when people use this?

[codymikol]

I actually was just about to make this feature request and to my surprise here it is 💯

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