string_ext: prefer snake_case to camelCase APIs.

* src/string_ext.lua (escapePattern, escapeShell, ordinalSuffix):
Rename from these...
(escape_pattern, escape_shell, ordinal_suffix): ...to these.
(M): Continue to support camelCase calls for backwards compatibility.
* specs/string_ext_spec.lua: Update.
Add specs to ensure camelCase APIs continue to work.