buffer.selection_n_start/end sets selection anchor/caret instead

[tmewett]

This is due to an upstream bug in Scintilla: Scintilla / Bugs / #2488 SCI_SETSELECTIONNSTART and END set anchor and caret instead

(I thought I'd open this to track the issue)

Originating discussion: How can I collapse selections to the start? · orbitalquark/textadept · Discussion #633