Update the customization type of `chatgpt-shell-language-mapping`

[swflint]

This improves the ability to edit this using customize

This is a first commit for #167.

That said, it may make more sense to consider a type as follows:

'(alist :key-type regexp
        :value-type function)

and use regular expression matching (thus only one entry for Objective C) to get the mode directly.

[xenodium]

Do you happen to have added more laguages to your config? Keen to contribute them to default?