Add documentation for JavaScript/TypeScript top-level await support

Document the new top-level await functionality for JavaScript and TypeScript
code execution in the Code Interpreter. This feature allows users to write
async code without manually wrapping it in async IIFEs, and automatically
returns the last expression's value.

Related to cloudflare/sandbox-sdk#261

Generated with Claude Code (https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: github-actions[bot]

src/content/docs/sandbox/api/interpreter.mdx

@@ -90,6 +90,38 @@ console.log(result.results[0].text); // "15"
 </TypeScriptExample>
 :::
 
+**Top-level await (JavaScript/TypeScript)**:
+
+JavaScript and TypeScript code automatically supports top-level `await` without requiring async function wrappers. The last expression's value is returned as the result.
+
+<TypeScriptExample>
+```
+const ctx = await sandbox.createCodeContext({ language: 'javascript' });
+
+const result = await sandbox.runCode(`
+const response = await fetch('https://api.example.com/data');
+const data = await response.json();
+data
+`, { context: ctx });
+
+console.log(result.results[0].json); // Parsed response data
+```
+</TypeScriptExample>
+
+Simple expressions are automatically returned:
+
+<TypeScriptExample>
+```
+const ctx = await sandbox.createCodeContext({ language: 'javascript' });
+
+await sandbox.runCode('const x = 10', { context: ctx });
+await sandbox.runCode('const y = 20', { context: ctx });
+
+const result = await sandbox.runCode('x + y', { context: ctx });
+console.log(result.results[0].text); // "30"
+```
+</TypeScriptExample>
+
 **Error handling**:
 
 <TypeScriptExample>