feat: add new target "moongres"

[XeniaLu]

This adds a new build target moongres, copying from the wasm-gc target, using the rustica-engine as runtime instead of moonrun, for landing future compiler/stdlib changes.

To test this PR, run:

cargo build --features moongres
cp target/debug/moon ~/.moon/bin/
cd ~/.moon/lib/core
moon test -v --target moongres --no-parallelize

Be sure to have the rustica-engine executable in your PATH.

Depends on:

• refactor: fix hard-coded TargetBackend usages moonbitlang/moon#1045
• fix: improve subprocess calls in moonbuild moonbitlang/moon#1047
• rustica-lang/rustica-engine@5fed3a1