GraalVM engine toolchains

[sgammon]

It would be great to expose GraalVM language engines, like js and wasm (GraalJs and GraalWasm) using Bazel's Toolchains feature. This would let other rules and downstream projects resolve these for their own purposes.

We can also register these language binaries, in some cases, as language-native toolchain types, as we've already done for Java toolchain use of GraalVM as a JVM. But we should probably treat this separately, since in some cases projects will need to resolve specifically the GraalVM implementations of these engines, and the regular toolchain system isn't really equipped to guarantee that.