Summary

This fixes a bug where the parser would panic when there is a "gap" in
the token source.

What's a gap?

The reason it's <= instead of just == is because there could be whitespaces between
the two tokens. For example:

#     last token end
#     | current token (newline) start
#     v v
def foo \n
#      ^
#      assume there's trailing whitespace here

Or, there could tokens that are considered "trivia" and thus aren't emitted by the token
source. These are comments and non-logical newlines. For example:

#     last token end
#     v
def foo # comment\n
#                ^ current token (newline) start

In either of the above cases, there's a "gap" between the end of the last token and start
of the current token.

Test Plan

Add test cases and update the snapshots.