Be more consistent in the documentation and variable declarations for base64 escaping vs unescaping:
* Move table documenting escaping conversions to be next to the internal escaping code that implements it. Mention it from the internal _un_escaping code (which reverses it) too.
* Centralize the two arrays which map from normal chars to escaped chars (k(WebSafe)Base64Chars), and make bidirectional documentation links between them and their reverse (unescaping) maps (kUn(WebSafe)Base64).
* Remove redundant list of escaping/unescaping methods in the public escaping.cc.

PiperOrigin-RevId: 517142667
Change-Id: I627e59a196ef855e5bf61ef7f4b509920e8d7acd