Feature: allow specifying start line for file display

[jonatanklosko]

Prerequisites

• I have searched for duplicate or closed feature requests
• I have read the contributing guidelines

Proposal

It would be useful to specify the start line for <File />, when showing a range of lines, rather than the whole file content.

Motivation and context

The docs mention startingLineNumber option, but it seems to not be implemented.

[amadeus]

you can sorta hack around this right now if you build a custom FileDiffMetadata object with a context only hunk (you'd have to use a diff component of course to render it, and just specify 'unified' for the diffStyle.

But it's something we should eventually support, just hasn't been a high priority at the moment