Add mailmap to consolidate author identities and correctly count contributions

[seisman]

With the new git shortlog command introduced in PR #3904, the number of contributions per author still differs from the counts shown on the GitHub contributors page. This discrepancy is due to variations in the commit author names or email addresses used over time.

This PR adds a .mailmap file (xref: https://git-scm.com/docs/gitmailmap) to resolve these inconsistencies automatically, avoiding the need to manually consolidate contributions from the same author.

With this file in place, the contribution counts now match exactly with those shown at https://github.com/GenericMappingTools/pygmt/graphs/contributors (Max is an exception, likely due to a name change from Meghan to Max a few years ago.

$ git shortlog -sne --group=author --group=trailer:co-authored-by | head -n 10
  1491	Dongdong Tian <[email protected]>
   626	Wei Ji Leong <[email protected]>
   427	Yvonne Fröhlich <[email protected]>
   293	Michael Grund <[email protected]>
   291	dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
   258	Leonardo Uieda <[email protected]>
   237	Will Schlitzer <[email protected]>
   203	Max Jones <[email protected]>
    55	Jiayuan Yao <[email protected]>
    17	actions-bot <[email protected]>

[seisman]

It seems setuptools_scm automatically exclude the .mailmap file from distributions, but I guess it does no harm to explicitly exclude it here.