Relates to #5853.
Two things happened here:
1. Construction of Mocks made an assumption that the selection should
always be deleted (outputTarget.ts:363). However, for NewContext and
PostKeystroke processes, we don't want to change anything.
2. Even if nothing is changed, the transcription would emit what is
in theory a no-op ruleTransform (insert="", deleteLeft=0,
deleteRight=0). But apps would treat this as deleting the selection.
This fix goes a little broader than I would have preferred, but adds a
readonly mode to the transcription and mock model, so that we can
control explicitly when changes are applied to the text store.