extension: add yosys_plugins attribute to orfs.default()

[oharboe]

Summary

Lets consumers expose out-of-tree yosys plugins (e.g. yosys-slang) without having to materialize a custom merged `yosys_share` tree.

• New `yosys_plugins` attribute on `orfs.default()` (extension.bzl)
• Plumbed through `global_config` as `CONFIG_YOSYS_PLUGINS` (config.bzl)
• New `_yosys_plugins` private attr on yosys-using rules (private/attrs.bzl)
• `yosys_environment` now sets `YOSYS_PLUGIN_PATH` from plugin dirnames; `yosys_inputs` includes the plugin files (private/environment.bzl)
• Old single-tree-artifact `yosys_share` path preserved as a fallback when no plugins are specified — existing consumers (including `test/downstream/`) keep working unchanged.

Motivation

OpenROAD's PR The-OpenROAD-Project/OpenROAD#10237 carries a custom `merge_yosys_share` rule that materializes a TreeArtifact merging `@yosys//:yosys_share` with a locally-built `slang.so`. @hzeller flagged it as fragile (manual `/bin/bash` shell-out, breaks on NixOS) and asked for the `pkg_files` pattern. `pkg_files` doesn't fit because bazel-orfs requires a TreeArtifact for `yosys_share` — but accepting plugins as a separate label_list sidesteps the merge entirely.

With this change, OpenROAD's `orfs.default()` becomes:
```python
orfs.default(
openroad = "//:openroad",
yosys_plugins = ["@yosys-slang//src/yosys_plugin:slang.so"],
)
```
and `bazel/merge_share.bzl` + `//:yosys_share_with_slang` are deleted.

Test plan

• Verified end-to-end against OpenROAD#10237: `bazelisk build //:openroad` clean, all 19 `//test/orfs/...` tests pass.
• CI green (this repo).