fix: Bazel 9 compatibility (bump to 0.22.0) and export toolchain_type

[nicdumz]

Fixes #278.

Bazel 9 compatibility (dep bump)

On Bazel 9 the source-built Jsonnet compilers fail to configure:

• The jsonnet 0.21.0 C++ sources' BUILD files (@jsonnet//core, @jsonnet//cmd) use bare cc_library / cc_binary / cc_test, which Bazel 9 removed and no longer auto-provides:

  This rule has been removed from Bazel. Please add a load() statement for it.

• jsonnet_go 0.21.0 pins rules_go 0.53.0, whose go/private/rules/cgo.bzl references the removed CcInfo symbol, so @go_toolchains//:all fails to load during toolchain resolution for any target (even when the cpp compiler is selected, because jsonnet_go is an unconditional dep).

Bumping jsonnet and jsonnet_go to 0.22.0 fixes both: the v0.22.0 C++ BUILD files already load("@rules_cc//cc:...", ...), and jsonnet_go 0.22.0 pulls rules_go 0.59.0, gazelle 0.47.0, and rules_cc — all Bazel-9 clean.

This is the same bump proposed in #272, #273, and #274, and validated green in #275 — thanks to those authors. This PR folds it together with the toolchain-type change below into one self-contained fix; happy to rebase and drop the bump hunk if one of those merges first.

Expose //jsonnet:toolchain_type

toolchain_type had no visibility, so it defaulted to private. A downstream module that wants to register its own Jsonnet toolchain (e.g. a prebuilt compiler binary, to avoid building the compiler from source) cannot reference @rules_jsonnet//jsonnet:toolchain_type without patching this repo. Making it public is consistent with the *_jsonnet_toolchain targets, which are already public.

Verification

On Bazel 9.1.0, examples/ builds and tests pass with both compilers, with no --incompatible_autoload_externally:

# cpp
bazel test //... --extra_toolchains=@rules_jsonnet//jsonnet:cpp_jsonnet_toolchain
#   -> 25 tests pass

# go (examples default compiler)
bazel test //...
#   -> 25 tests pass