Improve pathname field

[Jamiewarb]

See #90 and #94

We've integrated the Pathname field from @tinloof/sanity-kit, but it has some issues that I'd like to look at addressing in the future:

• When you create a new document that has a locked prefix, the prefix is editable. It locks upon changing fieldset or clicking in and out of the field
• When no pathname has been entered into a field with a locked prefix, the required validation error doesn't trigger (as it technically has a value). This needs a neat way to be handled, as it should be the same as required in that no value is there

We can look to contribute these to the sanity-kit package directly