Enhance error message verbosity

[Lai-YT]

What's the Problem?

To reproduce the issue, first, create a test file under the project root using the following command:

touch test.c
cat << EOF > test.c
int main() {
  return -1 * 2;
}
EOF

Next, compile the test file with VitaminC.
As of now, we don't yet support unary -, resulting in the "syntax error" message:

./vitaminc < test.c
# syntax error

As you can see, this error message lacks detail and doesn't provide insights into the actual error.

This pull request introduces an improvement by enhancing the error message verbosity:

./vitaminc < test.c
# syntax error, unexpected '-', expecting NUM or ID or '('

With this change, the error message is now more informative and helps in diagnosing the issue.

[leewei05]

LGTM! 🎸