MessageEventSource

Helper type for avoiding conflicts in types.

Uses the lib.dom.d.ts definition if it exists, otherwise defines it locally.

This is to avoid type conflicts between lib.dom.d.ts and @types/bun.

Unfortunately some symbols cannot be defined when both Bun types and lib.dom.d.ts types are loaded, and since we can't redeclare the symbol in a way that satisfies both, we need to fallback to the type that lib.dom.d.ts provides.