feat: expand tildes in config theme path to HOME

[preiter93]

What do you want to change?

When loading a theme from a path that includes a tilde:

theme="~/.cache/wal/colors-ghostty"

ghostty currently fails with the following error:

cannot include path separators unless it is an absolute path

I would like to expand the ~ of the theme.
If there is no ~ or expansion fails, it would fall back to the unexpanded value and just log.

I have a PR here: #13840

Why do you want to make this change?

I would like to be able to load themes from a path relative to my home directory.
This is relevant for example if you would like to use pywal to generate a ghostty theme.

This is not just relevant for me. Online I found this
https://gist.github.com/strlrd-29/c7dfed495d194fabc077956d7919c470
and the user used his full path. I think it would be more convenient and stable to just
usethe relative path.

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[jcollie]

!vouch