Add trailing line numbers to loops in Expr conversion

[c42f]

These trailing line numbers are used to attribute coverage to the end of the loop for parts of the loop header which run there after lowering.

At least, I think that's what's going on with the fact that coverage changes when using JuliaSyntax in Base.

[codecov[bot]]

Codecov Report

Merging #298 (215382d) into main (e7fe352) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #298   +/-   ##
=======================================
  Coverage   96.77%   96.77%           
=======================================
  Files          14       14           
  Lines        4090     4094    +4     
=======================================
+ Hits         3958     3962    +4     
  Misses        132      132           

Impacted Files	Coverage Δ
src/expr.jl	100.00% <100.00%> (ø)