Add a --selected-delimiter flag to gum choose (#937)

[renezander030]

Hi, and thanks for gum.

Per the contributing guide I am opening this as a Discussion before any PR.

gum choose --selected preselects items by splitting on the input delimiter (comma by default). If an option value itself contains a comma, it cannot be preselected, because the same delimiter splits both the option list and the selected list.

Proposal: add a --selected-delimiter flag that defaults to the current behavior, so the selected list can use a different separator than the options. Existing scripts keep working, and it only matters when an option contains the delimiter.

This tracks #937. If you are open to it, I can send a small PR with the flag and a test, and adjust the flag name or default to match your preference.

Rene