Replace deprecated default initializations with decl literals

[linusg]

Follow-up to 0b9fccf.

[linusg]

ah, I suppose this needs a zig1.wasm update to work...

[mlugg]

Yep, that's why I held off on doing it in that PR. However, this work is still valuable, so keep it around! Once we've done a zig1.wasm update we can merge this.

By the way, I haven't looked over all of the changes here, but InternPool shouldn't use default values at all -- instead, it should probably expose a pub const empty: InternPool = .... That's because many of the field values are tied together.

[mlugg]

@linusg#21367 contained a zig1.wasm update, so you should be good to go ahead with this branch now if you rebase!

[linusg]

Rebased, did a quick sweep of newly added = .{}, added InternPool.empty in a separate commit.