feat: expose Ruby engine configuration

[p0deje]

Exposes Ruby engine targets to select() depending on installed Ruby interpreter:

BUILD:

rb_library(
    name = "my_lib",
    srcs = ["my_lib.rb"],
    deps = select({
        "@ruby//engine:jruby": [":my_jruby_lib"],
        "@ruby//engine:truffleruby": ["//:my_truffleruby_lib"],
        "@ruby//engine:ruby": ["//:my__lib"],
        "//conditions:default": [],
    }),
)