Add a feature to test whether Stardoc support module extensions.

[phst]

See https://github.com/bazelbuild/stardoc/issues/192.

See logic in https://github.com/phst/rules_elisp/blob/8606695e977b876ffb38f881f54e50de1606a733/docs/BUILD for prior art.

[phst]

In https://github.com/phst/rules_elisp/blob/8606695e977b876ffb38f881f54e50de1606a733/docs/BUILD I selectively build the extensions target (for a Starlark file containing module extensions) depending on whether the current Bazel version is 7 or not. Whether this works or not appears to depend mainly on the Bazel version, at least for recent enough Stardoc versions. Happy to switch to a better approach if there's one!

[fmeum]

There is, you can detect the required Bazel feature directly: https://github.com/bazelbuild/stardoc/blob/bf441c86274770ea281d21e22292ecdaa6451eaa/stardoc/stardoc.bzl#L88

Let me know if that doesn't work and we can reconsider this.

[phst]

It works, thanks! https://github.com/phst/rules_elisp/commit/57787964fda190305fc7353e618da089ed3aff4b