Allow targeting of specific file extensions for babel rollup plugin

[kylemh]

Current Behavior

I'm unable to customize the targeted file extensions for plugins and don't have an easy way to ignore one plugin and extend it how I want it. I need to do so to resolve this issue since I need to have babel run across .svg files to avoid bugs in Edge for my icon library.

Desired Behavior

Allow me to determine the file extensions targeted by each rollup plugin.

[agilgur5]

Sorry I didn't respond earlier, needed to be at my computer so I could add links and easier and this one's gonna be backlogged anyway unfortunately.

Basically I don't think it makes sense to add a specific feature for this, but I've been planning (and at some point Jared did too, c.f. #389 (comment)) on opening up the Rollup config since a bit before #389 so that anyone can more easily override any plugin's config as necessary. More concretely, that would be something like a deep merge or new functions added in tsdx.config.js, one for each plugin.

That would support this and a variety of things as a generic case, without adding the maintenance cost of a specific feature for this very specific use-case. So this effectively duplicates #389 in that sense, and in the meantime, I'd suggest a similar fix as there, using patch-package for your needs or a really hacky tsdx.config.js override with the current set-up.

Also related to #635