[mime]: support custom MIME types

[2dragonly]

Since experimental (x-) and vendor-specific (vnd.) MIME types are excluded, please support adding custom MIME types.

We need a way to register them globally so they are available package-wide:

// index.ts
import { mimeTypes } from "@remix-run/mime"

mimeTypes["swf"] = "application/x-shockwave-flash"

// detect.ts
import { detectContentType } from "@remix-run/mime"

detectContentType("swf") // Should detect the new type

This should also ensure the new types are detected correctly in other packages like @remix-run/response