config: move optional path parsing into RepeatablePath

This commit refactors RepeatablePath to contain a list of tagged unions
containing "optional" and "required" variants. Both variants have a null
terminated file path as their payload, but the tag dictates whether the
path must exist or not. This implemenation is used to force consumers to
handle the optional vs. required distinction.

This also moves the parsing of optional file paths into RepeatablePath's
parseCLI function. This allows the code to be better unit tested. Since
RepeatablePath no longer contains a simple list of RepeatableStrings,
many other of its methods needed to be reimplemented as well.

Because all of this functionality is built into the RepeatablePath type,
other config options which also use RepeatablePath gain the ability to
specify optional paths as well. Right now this is only the
"custom-shaders" option. The code paths in the renderer to load shader
files has been updated accordingly.

In the original optional config file parsing, the leading ? character
was removed when paths were expanded. Thus, when config files were
actually loaded recursively, they appeared to be regular (required)
config files and an error occurred if the file did not exist. **This
issue was not found during testing because the presence of the
"theme" option masks the error**. I am not sure why the presence of
"theme" does this, I did not dig into that.

Now because the "optional" or "required" state of each path is tracked
in the enum tag the "optional" status of the path is preserved after
being expanded to an absolute path.

Finally, this commit fixes a bug where missing "config-file" files were
not included in the +show-config command (i.e. if a user had
`config-file = foo.conf` and `foo.conf` did not exist, then `ghostty
+show-config` would only display `config-file =`). This bug applied to
`custom-shaders` too, where it has also been fixed.

src/config.zig

@@ -23,6 +23,7 @@ pub const OptionAsAlt = Config.OptionAsAlt;
 pub const RepeatableCodepointMap = Config.RepeatableCodepointMap;
 pub const RepeatableFontVariation = Config.RepeatableFontVariation;
 pub const RepeatableString = Config.RepeatableString;
+pub const RepeatablePath = Config.RepeatablePath;
 pub const ShellIntegrationFeatures = Config.ShellIntegrationFeatures;
 pub const WindowPaddingColor = Config.WindowPaddingColor;