Expand Unity importer support

[ruccho]

Rust

rust-analyzer supports non-cargo projects with rust-project.json.
We would generate rust-project.json at the root of Unity project like *.csproj.
https://rust-analyzer.github.io/manual.html#non-cargo-based-projects

We would be able to manage our own dependencies without using cargo. We would treat each Rust source as a crate root (which we would like to be configurable by the user) and make a build system that would call rustc directly.

Support for the Component Model could also be achieved by extending the above approach.

MoonBit

MoonBit currently only provides integrated build system with moon build.
https://moonbitlang.github.io/moon/

It seems not embeddable enough but its source is open.
https://github.com/moonbitlang/moon

Others (TBD)