feat(cmake): add installation support for pkg-config dependency detection (#4077)

* add installation support for pkg-config dependency detection

pkg-config is a buildsystem-agnostic alternative to
`pybind11Config.cmake` that can be used from build systems other than
cmake.

Fixes #230

* tests: add test for pkg config

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

Co-authored-by: Henry Schreiner <henryschreineriii@gmail.com>

.pre-commit-config.yaml

@@ -12,6 +12,9 @@
 #
 # See https://github.com/pre-commit/pre-commit
 
+# third-party content
+exclude: ^tools/JoinPaths.cmake$
+
 repos:
 # Standard hooks
 - repo: https://github.com/pre-commit/pre-commit-hooks