Adding inserting variants for 'm', 't', 'r',etc. about add-ed HOT 3 CLOSED

Of special note is that the 'R' variant could both be a prepending version of 'r' and a modified version of 'R' which captures only stderr output from any shell escape run. I'm unsure if the latter is that desired, since shell tools exist to solve that problem, but it shows that I might build myself into a corner if I add variant commands too freely.

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Since the advantage of inserting variants is generally dubious it is best to not add any more than are considered standard in Ed. Most have been disabled, but the code still exists and they can be enabled easily. This issue can be used to track any related discussion.

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Added in POTENTIAL_FEATURES.md, abandoned until requested.

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