Enable syntax highlighting for *.tmpl on GitHub (#13571)

Go templates are syntactically very similar to handlebars, this enabled
their syntax highlighting on GitHub.

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

.gitattributes

@@ -1,3 +1,4 @@
 * text=auto eol=lf
 /vendor/** -text -eol linguist-vendored
 /public/vendor/** -text -eol linguist-vendored
+/templates/**/*.tmpl linguist-language=Handlebars