Problem

Suggested in #29646 (comment)

In addition to the single-char (and in rare cases int) marker definitions, we should also support long names "o" -> "circle". These long names should be supported wherever the short mnemonics are, with the exception of the fmt string syntax in plot() - that should still only accept single chars.

This allows more readable code marker="tri_up" instead of marker="2". It also opens a route for deprecating the int markers, which create trouble for parsing/validating.

Proposed solution

The long names should mostly be the ones from the "description" column in https://matplotlib.org/devdocs/api/markers_api.html#module-matplotlib.markers, with some slight adjustments:

• "filled_plus" instead of "plus (filled)"
• do not include "nothing" - we already have more than enough no-marker definitions.