fix: shadow global Symbol in symbol/async-iterator

The JavaScript linting CI job failed on develop:

  lib/node_modules/@stdlib/symbol/async-iterator/lib/main.js
    42:65  error  The 'Symbol.asyncIterator' is still an experimental
    feature and is not supported until Node.js 10.0.0. The configured
    version range is '>=0.12.18'  n/no-unsupported-features/es-builtins

Root cause: main.js referenced the global Symbol directly. The
n/no-unsupported-features/es-builtins ESLint rule flags
Symbol.asyncIterator as requiring Node >= 10.0.0, which exceeds
the project configured minimum of >=0.12.18. Shadowing the global
with var Symbol = require( '@stdlib/symbol/ctor' ) removes the
global reference and resolves the violation. The local binding
resolves to the native Symbol in environments where it exists,
preserving runtime behavior identically.

The sibling package symbol/to-primitive uses the same pattern for
Symbol.toPrimitive, which is also flagged by the same rule.

Failing run: https://github.com/stdlib-js/stdlib/actions/runs/25195861572

https://claude.ai/code/session_01KcDPB7otMv5FFbBc5N12mu

Author: Planeshifter

lib/node_modules/@stdlib/symbol/async-iterator/lib/main.js

@@ -21,6 +21,7 @@
 // MODULES //
 
 var hasAsyncIteratorSymbolSupport = require( '@stdlib/assert/has-async-iterator-symbol-support' ); // eslint-disable-line id-length
+var Symbol = require( '@stdlib/symbol/ctor' );
 
 
 // MAIN //