feat: support DisableBuiltin("$env") to hide the $env variable

[boinger]

Summary

$env is a hardcoded special variable with custom handling in the checker and compiler. It's not in the Builtins map, so DisableBuiltin("$env") previously had no effect.

This PR checks config.Disabled["$env"] before each $env special-case handler. When disabled, $env falls through to normal identifier resolution, which errors in strict mode as "unknown name $env". This is consistent with how DisableBuiltin works for regular builtins.

Changes

• checker/checker.go: Guard $env handling in identifierNode, memberNode, callNode, and callBuiltin (the get($env, ...) path) with !v.config.Disabled["$env"]
• compiler/compiler.go: Guard OpLoadEnv emission in IdentifierNode with !c.config.Disabled["$env"]
• test/issues/710/issue_test.go: 7 subtests covering all disabled paths (standalone, member access, call, get()), non-strict mode behavior, and regression tests for default behavior

Use case

Users embedding expr as a DSL may have domain variables that conflict with $env, or may want to restrict access to the full environment object for sandboxing.

Fixes #710

[Copilot]

Pull request overview

This PR makes the hardcoded special variable $env respect DisableBuiltin("$env"), allowing users to hide $env (e.g., to avoid naming conflicts or to restrict environment access) and have it fall back to normal identifier resolution.

Changes:

• Guard $env special-case handling in the checker so it only applies when $env is not disabled.
• Guard $env bytecode emission (OpLoadEnv) in the compiler so it only applies when $env is not disabled.
• Add regression tests covering $env disabled/enabled behaviors across identifier, member access, call, and get($env, ...) paths.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
checker/checker.go	Adds !config.Disabled["$env"] guards around $env special handling so disabling causes normal name resolution/errors.
compiler/compiler.go	Prevents OpLoadEnv emission when $env is disabled, aligning runtime behavior with checker resolution.
test/issues/710/issue_test.go	Adds coverage for strict and non-strict behavior when $env is disabled, plus default enabled regressions.

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