Add full statement range support to roxygen @examples and @examplesIf

[DavisVaughan]

Addresses posit-dev/positron#8559 (via the fact that we no longer try to run examples one line at a time, there is still a regression in the frontend code logged elsewhere)

I can't find an issue that talks about wanting full Statement Range support in roxygen comments, but since we have the regression where running one line at a time doesn't work either, this became a much higher priority for me.

---

This PR adds full Statement Range support to roxygen @examples and @examplesIf blocks.

• Previously we could only run one line at a time (and a regression had broken this too)
• Now we can run entire blocks of code at once, just like the regular statement range provider

It works by extracting out the @examples or @examplesIf blocks into a subdocument, which we parse on its own and run through the statement range provider (adjusting row positions accordingly).

This is such a big QOL improvement for me 😬

I've also reworked the tests for statement range so that you can debug them one at a time. It was pretty frustrating to debug one individual test when others were in the way. Along the way I made it so that point_from_cursor() could accept an alternative cursor token than @, because that's what is used to denote roxygen tags! This allowed me to make the statement range tests much more readable.

Screen.Recording.2025-11-19.at.12.28.37.PM.mov