Improvements to `english_irregulars.csv`

Merged this with another irregular verbs spreadsheet I use for studying
overregularization errors, and resolved any resulting inconsistencies.
There is now. To identify verbs to be excluded for this study, the
following should work (not run):

    verbs.to.include <- subset(this.data, nchar(as.character(exclusion))
== 0 & no.change == FALSE & auxiliary == FALSE)