Raise syntax error when `\` is at end of file (#17409)
## Summary
This PR fixes a bug in the lexer specifically around line continuation
character at end of file.
The reason this was occurring is because the lexer wouldn't check for
EOL _after_ consuming the escaped newline but only if the EOL was right
after the line continuation character.
fixes: #17398
## Test Plan
Add tests for the scenarios where this should occur mainly (a) when the
state is `AfterNewline` and (b) when the state is `Other`.
