Refactor: libcrmcommon: New PCMK__VALUE_EN string constant

There's no reason to make this public because:

1. It's not enumerated in any schema.
2. According to the OCF spec, the "lang" attribute's value must be a standard language identifier ("BCP 47" https://www.rfc-editor.org/rfc/bcp/bcp47.txt). It will never be feasible for us to enumerate all of them. Clients shouldn't rely on Pacemaker string constants for these values.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>