frontends: support several extensions

[terrablue]

It might be desirable to support several extensions, as in @primate/react supporting both .jsx and .tsx, to allow for a transitional period from untyped to typed components, without being forced to change an entire application at once. This is in line with Primate's goal of having flexibility in moving from one technology to another.

Currently, frontend components accept an extension string config property with a sane default (.jsx for React, .svelte for Svelte, etc.). This will need to be changed to an array of extensions, as in [".jsx", ".tsx"], which is more complex to validate for errors.

We would also need to detect files with the same name but different extension, to potentially support extensionless imports like import Component from "#component/PostView" in the future.

Due to the low return:investment ratio, this will be low priority if and until enough interest in expressed.