While the interfaces for setting number and boolean options are now unified by #25394, there is still a separate `set_string_option` function that is used for setting a string option. This PR removes that function and merges it with set_option.
BREAKING CHANGE: `v:option_old` is now the old global value for all global-local options, instead of just string global-local options. Local value for a global-local number/boolean option is now unset when the option is set (e.g. using `:set` or `nvim_set_option_value`) without a scope, which means they now behave the same way as string options.
Ref: #25672
e19cc9c9b7
