Correct annotation calculation with annot substr

The commit e1a76bb was ultimately
reverted due to it seeming to introduce an abort signal in CI.

I suspect this could be connected to the code in the StyledStrings
stdlib that has an @inbounds next to an assumption that substring ranges
are valid (regioniterator.jl:13).

The annotation calculation for SubStrings was incorrect, as it didn't
take into account the substring offset or width. CI will hopefully tell
us if this is the root cause.

base/strings/annotated.jl

@@ -370,8 +370,13 @@ See also: `annotate!`.
 """
 annotations(s::AnnotatedString) = s.annotations
 
-annotations(s::SubString{<:AnnotatedString}) =
-    annotations(s, s.offset+1:s.offset+s.ncodeunits)
+function annotations(s::SubString{<:AnnotatedString})
+    map(function ((region, annot),)
+            region = max(1, first(region) - s.offset):min(s.ncodeunits, last(region) - s.offset)
+            (region, annot)
+        end,
+        annotations(s.string, s.offset+1:s.offset+s.ncodeunits))
+end
 
 function annotations(s::AnnotatedString, pos::UnitRange{<:Integer})
     # TODO optimise