Allow "synchronous" loading of cartridge files

[cute-the-niini]

In general, cartridge files live only in the kernel side, and cartridge processes must use an asynchronous RPC call to read them. This generally works for games made exclusively for Kate, however existing engines have varied needs of reading data "synchronously" (at least from the perspective of the game's code), so we need to support that as well.

This patch adds a way of pre-loading certain assets at the time the cartridge sandbox is being built, such that all reads of these assets from the cartridge's process appear to be synchronous (since they're moved to the cartridge's process at the time the sandbox is built). This in turn lets us support games written in RPG Maker MV and Godot 3 and 4 with no changes required on the game's code. The environment just fulfils all of the game's code expectations instead.

Note that Godot 4 games are not yet enabled in Kate due to the security vulnerability described in https://github.com/orgs/qteatime/discussions/1 --- cartridge processes need to be fully isolated before Godot 4 games can be enabled, as their processing requirements make it possible for cartridges to exploit Spectre vulnerabilities to read privileged information from the Kate kernel process, at least in theory.

[cute-the-niini]

• Godot needs a bridge to disable serviceWorker as well, since otherwise the runtime code will trap by trying to read navigator.serviceWorker inside of a browser sandboxed process.