Map ignore files to Git Ignore syntax

This would catch any dotfiles that end in "ignore", but not `.ignore`

Set as a lower priority rule (90 vs the usual 50) because it's a blanket
assignation. If there are conflict with other, more specific rules,
those will take precedence.

src/syntax_mapping/builtins/common/90-ignore-files.toml

@@ -0,0 +1,2 @@
+[mappings]
+"Git Ignore" = [".?*ignore"]