FoldedLineWriter: fix folding inside surrogate pairs

[blendmaster]

Tricky bug, since it only happens if you happen to have a surrogate pair right at the line boundary.

[codecov-io]

Current coverage is 76.13%

Merging #36 into master will not affect coverage as of efa2224

@@            master     #36   diff @@
======================================
  Files          203     203       
  Stmts         7374    7378     +4
  Branches      1141    1143     +2
  Methods                          
======================================
+ Hit           5614    5617     +3
- Partial        237     238     +1
  Missed        1523    1523       

Review entire Coverage Diff as of efa2224

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[mangstadt]

Thank you, @blendmaster.