fix: improvements and ipc fixes for loading window content using custom protocol

[amrbashir]

closes #5478

• made distDir and devPath optional so if someone wants to load all their windows using custom protocols they don't need to specify and create an empty directory for distDir when building
• added WebviewUrl::CustomProtocol which is the same as WebviewUrl::External but more explicit.
• fixed ipc calls from a window that loaded over custom protocol

[amrbashir]

@lucasfernog what use-case is being covered by this check?

[lucasfernog]

IIRC it's for mobile.. should've documented it my bad

[amrbashir]

won't this be covered by the tauri::// case as well?

[amrbashir]

There is one more improvement we can make but don't know if it makes sense? currently users will need to always explicitly specify the url for each window either in code or in tauri.conf.json, would it make sense if we allowed them to set a base url for all windows instead of tauri:// which is the default for all windows at the moement?

[lucasfernog]

There is one more improvement we can make but don't know if it makes sense? currently users will need to always explicitly specify the url for each window either in code or in tauri.conf.json, would it make sense if we allowed them to set a base url for all windows instead of tauri:// which is the default for all windows at the moement?

I think this would complicate the codebase and introduce some bugs likely.. let's keep that until someone actually requests it

[amrbashir]

I agree