fix: update nginx config file parsing logic to handle variables with …

…braces

Address 43818 so that the user can include variables with braces in the 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 for checking the parser can correctly parse variables with braces
fix the messy test case in lex_test.go since the values that are separated by spaces need to be tokenized
fix the existing linter issues, which includes updating the deprecated functions, improving readability of variable names, etc.

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

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

nginxinc / nginx-go-crossplane