Update to latest google3 version

* Remove glog and gflags dependencies; use abseil-cpp instead.
  * Use ABSL_LOG macros instead of S2_LOG
  * Use ABSL_CHECK macros instead of S2_CHECK
* Require abseil-cpp LTS 20240116
* Remove Python SWIG.  (This will be maintained in the github branch.)
* Add new files:
  * s2chain_interpolation_query
  * s2density_tree
  * s2gmock_matchers
  * s2fractal
  * s2shapeutil_count_vertices
  * s2shapeutil_edge_wrap
* Remove S2Shape::shape(int)
* Add new predicates in s2predicates.h
* Make S2Region::GetCellUnionBound pure virtual
* Add new S2Shape functions

Author: jmr

.clang-format

@@ -0,0 +1,7 @@
+---
+Language: Cpp
+BasedOnStyle: Google
+# Always use `T* p`.
+DerivePointerAlignment: false
+PointerAlignment: Left
+...