Enable check-symlinks pre-commit hook

Rationale:

- Small but likely benefit in general, since there are no currently
  foreseen intentional use cases of committing of broken/dangling
  symlinks in this project. So such symlinks that arise are likely
  unintentional.

- If the end-of-file-fixer hook has run on a Windows system where
  `core.symlinks` has *not* been set to `true`, and symlinks' paths
  have not been excluded, then a newline character is added to the
  end of the path held in the regular file Git checks out to stand
  in for the symlink. Because it is not actually a symlink, this
  will not detect the problem at that time (regardless of the order
  in which this and that hook run relative to each other). But when
  it is then run on CI on a system where symlinks are checked out,
  it will detect the problem.

Author: EliahKagan

.pre-commit-config.yaml

@@ -27,6 +27,7 @@ repos:
   hooks:
   - id: end-of-file-fixer
     exclude: ^test/fixtures/
+  - id: check-symlinks
   - id: check-toml
   - id: check-yaml
   - id: check-merge-conflict