Remove escaped mac/windows eol from AST string value (#7724)
## Summary
This PR fixes the bug where the value of a string node type includes the
escaped mac/windows newline character.
Note that the token value still includes them, it's only removed when
parsing the string content.
## Test Plan
Add new test cases for the string node type to check that the escapes
aren't being included in the string value.
fixes: #7723
