fix: update parsing logic to handle variables with braces

[sylwang]

Proposed changes

Address 43818 so that the user can use variables with braces in the nginx config file.

• update lex.go to correctly parse variables like ${foo}
• add an nginx.config file in testdata for a test case of variables with braces
• add a test case in parse_test.go to ensure that the parser can correctly parse variables with braces
• fix the "messy" test case in lex_test.go so that values separated by spaces are tokenized

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md)