Svelte 5: Reintroduce `frame`/`pos`/`toString()` in compiler warnings

[Conduitry]

trafficstars

Describe the problem

Svelte 4 has some convenient fields on warnings that display the information in a human-readable way - but which are missing in Svelte 5. The frame contained some nearby context in the file and had a caret pointing at the position of the warning. The .toString() method was somewhat customized per type of warning, and would display some combination of the message, the filename, the line/column, the nearby code in the component, etc.

Describe the proposed solution

The simplest for consumers would be for the frame, pos, and toString() to be reimplemented as they were in v4, but there are other options here, too.

Importance

nice to have