externals: Add catch2 v3.2.1

Merge commit '6879e5bb1c' as 'externals/catch'

externals/catch/.gitattributes

@@ -0,0 +1,22 @@
+# This sets the default behaviour, overriding core.autocrlf
+* text=auto
+
+# All source files should have unix line-endings in the repository,
+# but convert to native line-endings on checkout
+*.cpp text
+*.h text
+*.hpp text
+
+# Windows specific files should retain windows line-endings
+*.sln text eol=crlf
+
+# Keep executable scripts with LFs so they can be run after being
+# checked out on Windows
+*.py text eol=lf
+
+
+# Keep the single include header with LFs to make sure it is uploaded,
+# hashed etc with LF
+single_include/**/*.hpp eol=lf
+# Also keep the LICENCE file with LFs for the same reason
+LICENCE.txt eol=lf