Make the character selection boundary configurable

[JimBarber]

Hi.

I was wondering if the characters used for selection boundary can be made to be configurable via the config file?

At the moment they are hard-coded in the boundary constant of the selectWord function in src/terminal/Screen.zig
There have been a number of issues in the past asking for adjustments to this selection boundary.
e.g.

• Selection boundaries should include '$' #2933
• Word selection is too greedy #904
• Word selection behaves unexpectedly #741

There's precedence for this to be configurable in other terminals.
e.g.
For xterm the default boundary character(s) can be overridden via the charClass resource (and the -cc command line option).
For Gnome Terminal the word-char-exceptions property for the terminal profile can be configured using gsettings or dconf.
For kitty the select_by_word_characters option can be set in kitty.conf to set the boundary characters.

I suspect no one can agree on what a good value for the selection boundary characters are, which is why it would be good if your constant provides what you consider to be sane defaults, and then allow the user to override that.
I personally can't make the switch to ghostty due to the selection when I double click words not working how I want it to (it still selects too much for my tastes, I want / to also be in the list so I can easily select partial paths and then drag out from there if I want more, but I know that doesn't fit with the idea of selecting URLs that I rarely need).

Thanks for your consideration.

[pluiedev]

Dupe of #4263