transcriptions-en: support the FOOT-PUT split

The modern FOOT vowel is a result of:
1.  a FOOT-GOOSE split;
2.  a PUT-CUT split;
3.  a FOOT-PUT merger.

The pre-split forms (as found in the Irish and Scouse accents) can
be reconstructed by creating an artificial FOOT-PUT split (i.e. by
reversing the splits and mergers described above).