[0xhackify]

Closes #192

Adds an env(key, optionalDefault) helper that returns the value of an
environment variable, or the optional default when it is not set.

• Uses os.LookupEnv, so a variable set to "" returns "" (not the default)
• Added TestEnvDSLFunction, updated the signatures test and the README

Testing: go test -run TestEnvDSLFunction -v passes.
Note: TestDslExpressions has 4 pre-existing failures (gzip, zlib, zip,
deflate) on Go 1.27.3; they also fail on main without this change,
likely due to compress/flate output changes in newer Go versions.

Security note: since templates may come from third parties, nuclei may
want to gate this behind an opt-in flag (similar to -env-vars) to avoid
leaking secrets from the scanning machine.

[Mzack9999]

Closing this. Putting env() in the default DSL helpers would let a template read the process environment, including API keys, and send those values out in a request. I do not want that in the default set. Happy to look again if it stays out of the helpers a template can call.