Add absolute source offsets to parsed selector nodes - #341

Sorry for the wait. Your draft note says you opened it for review, and that review is what was missing, not anything on your side.

I rebuilt it on current main: it merges cleanly and npm test passes with 1002 tests. Then I diffed every node's position against main across six selector shapes. The offsets are purely additive everywhere, with one exception:

.#{$c}    class "#{$c}"    sourceIndex 4 -> 0

That is the right value, and it is what #243 asks for, so it is welcome. But it changes an existing property, the description does not mention it, and no test pins it, so as it stands it reads as a side effect rather than a fix.

Two things before this leaves draft:

• Call the sourceIndex change out in the description, and add a test for it.
• source.start.column stays at 5 for that same node, where Wrong source and sourceIndex for Sass interpolated selectors #243 expects 1, so the issue is only half closed. Either finish it here, or say in the PR that the column is out of scope and Wrong source and sourceIndex for Sass interpolated selectors #243 stays open for it.

With those, I will take it.

One practical note: main has moved a fair amount since September. 7.1.7 and 7.1.8 are out, and several PRs have landed, including work in splitWord and the source helpers. You will want to rebase before the final pass.