Add a way to get the span for an interface's identifier

[awareness481]

For some errors you may want to have the span of only the interface's identifier

interface A {}
          _

instead of the whole interface

interface A {}
_____________

currently you can only do the latter

          I think we should add a field like `name_span` to the `Interface` type

Originally posted by @kdy1 in https://github.com/dudykr/stc/pull/985#discussion_r1167311338