File references can't name a range of lines

[Brechard]

Foo.ts:20-40 doesn't become a link, and neither does GitHub's Foo.ts#L20-L40. Only :12 and :12:5 are recognised, so a reference to a block of code either loses the range or isn't clickable at all.

Two things get in the way: the position suffix pattern doesn't allow a range, and the external-scheme check reads :20-40 as a URI scheme and bails out early.

Worth having because the panel already knows how to reveal a line — highlighting a range is the same mechanism applied to more than one row.