Feature request: marker-based selection

The ability to use Shift+arrows etc to select text is arguably one of the best features of micro. The problem is that it's very much contingent on terminal support, and some terminals don't provide any means of detecting modifiers on navigation keys at all (e.g. FreeBSD vt).

Currently, working around this requires rebinding a bunch of selection commands to some arbitrary keys; however, when those keys aren't navigation keys + modifier because such aren't supported by the terminal, it's nearly impossible to remember those bindings.

The traditional way to handle selection in Unix text-mode editors is to use selection markers a la WordStar - basically a single keybinding to enter selection mode, in which all navigation keys without modifiers select stuff; and another keybinding to exit that mode, accepting current selections. Some editors, like mcedit, use a single keybinding for both operations.

It would be nice to have this as a bindable command in micro, for use with those funky terminals without having to abandon micro entirely.

Start by tracing micro's existing selection commands, navigation-key handling, and bindable command definitions. Check how terminal modifier input is represented, especially for FreeBSD vt, and clarify marker-mode entry and exit semantics. Done means users can bind the mode and navigation keys select text without terminal modifiers.