Add .gitattributes forcing LF line endings for all text files

Test Windows was failing every matrix entry near-instantly with no
test output and no error detail beyond a generic "exit code 1" --
consistent with the runner's default core.autocrlf=true silently
turning the composite action's embedded bash script into CRLF on
checkout, which corrupts it for git-bash (e.g. a stray \r gets
appended onto the last token of a line, like the ds2.exe path). With
no .gitattributes in the repo, checkout line endings were entirely at
the mercy of whatever autocrlf setting a given runner happens to
default to.

Author: compnerd

.gitattributes

@@ -0,0 +1 @@
+* text=auto eol=lf