mailmap: update info for wchargin (#6939)

Hello! I used to work on TensorBoard full time. Then I changed my name.
(👋!) I still consult this repo sometimes, so I'd like for my name to be
updated and correct.

This patch adds a [Git mailmap], which is a plain text file that teaches
Git to correct errors or duplication in user information. It
transparently fixes the output of commands like `git shortlog` and `git
show`, without needing to rewrite old hashes or destroy any information.
I've initialized the mailmap with my name and also unified the email
addresses to the one that I used most commonly (99.2% of the time).

Test Plan:
`git shortlog -nse | head` and see that my name is [spelled right
now][wc].

[Git mailmap]: https://git-scm.com/docs/gitmailmap
[wc]: https://wchargin.com

.mailmap

@@ -0,0 +1,3 @@
+Willow Chargin <[email protected]>
+Willow Chargin <[email protected]> <[email protected]>
+Willow Chargin <[email protected]> <[email protected]>