fix: rewrite stripSharedIndent to fix some correctness issues (#602)
fix: rewrite stripSharedIndent to fix some correctness issues (#602)

Progress toward #125

The original code tried to emulate CoffeeScript's herestring
whitespace-stripping rules, which have some bizarre edge cases, but that's not
needed anymore since coffee-lex is responsible for copying that behavior.
`stripSharedIndent` also had some bugs that I was running into in another
commit, where in some edge cases it would remove non-whitespace contents. Rather
than trying to directly fix those issues, I just rewrote the function in a way
that I think is simpler.
eventualbuddha/decaffeinate,decaffeinate/decaffeinate,alangpierce/decaffeinate,alangpierce/decaffeinate,decaffeinate/decaffeinate,alangpierce/decaffeinate,eventualbuddha/decaffeinate