fix: do not push expectation errors to EOF

[juleswritescode]

Splitter Context: If we expect a following keyword, but the next token is EOF, we'd push the expectation error onto the EOF token.

That EOF token is the last token in the kind vector, so i == kind.len() - 1, so self.start[i+1] would error:

To solve this case, we'll push the expectation error on the last token before EOF.

[psteinroe]

oh wow, great catch!