Add InterpolationDelimiter to the list of "string or delimiter"

Unsure if this won't break anything else :/.
WIP Retrying s:Match function

The s:Match function tries to find a pattern, but if the found one is in
a string or comment, it fails. However, it's possible that beyond the
string, there's a comma or something else that *also* fits the pattern
and will not be found.

There seems to have been an attempt to fix this issue by matching in
reverse with the unused `s:MatchLast` function, but it's not really used
anywhere, so it was removed.

The current implementation tries to match the same pattern on the rest
of the string after failing once.

Tests and documentation are needed. There may also be issues with
multibyte, not sure.
Don't match "line", match "lnum"

How did this ever work?
Tests for string interpolation problem