Vim mode: change commands and insert mode reworked

What

Vim mode lets you edit your prompt with vim-style keys. How it enters insert mode, where typing adds text, has been reworked:

• The change operator deletes the chosen text, keeps it in the register (vim's clipboard), and calls enterInsert at the start of that range. Before, it inserted replacement text and moved the cursor itself.
• enterInsert applies NFC normalisation and places the cursor by whole visible characters. NFC normalisation stores accented letters in one standard form.
• setText passes the previous text and cursor position to the input.
• Dot-repeat (repeating the last change with .) and the visual-mode change path share the same text setter.

Why

Change commands now behave more like real vim. The character handling likely fixes cursor jumps and undo problems when typing accented or combined characters in insert mode.