fix(cli): handle edge cases around `export`s in doc tests and default export (#25720)

This commit fixes issues with the pseudo test file generation logic,
namely:

- `export`s declared in snippets
- auto import insertion for `default export`

## Case 1: `export`s declared in snippets

In the previous implementation, `export`s declared in snippets were
moved to the top level of the module in the generated pseudo test file.
This is required because `export` must be at the top level.

This becomes a problem if such a `export` has a body, containing a
reference to a local variable. Suppose we extract this snippet from
JSDoc:

```ts
const logger = createLogger("my-awesome-module");

export function sum(a: number, b: number): number {
  logger.debug("sum called");
  return a + b;
}
```

This gets converted into the following invalid code (note that `export
function sum` is moved to the top level, but its body references
`logger` variable which can't be referenced from here):

```ts
export function sum(a: number, b: number): number {
  logger.debug("sum called");
  return a + b;
}

Deno.test("./base.ts$1-7.ts", async () => {
  const logger = createLogger("my-awesome-module");
});
```

To resolve this issue, this commit adds a logic to remove the `export`
keyword, allowing the exported items to stay in the `Deno.test` block
scope, like so:

```ts
Deno.test("./base.ts$1-7.ts", async () => {
  const logger = createLogger("my-awesome-module");

  function sum(a: number, b: number): number {
    logger.debug("sum called");
    return a + b;
  }
});
```

## Case 2: default export

Previously `default export foo` was not captured by the export
collector, so auto import insertion didn't work for this case. To put it
concretely, the following code snippet didn't work when run with `deno
test --doc` because `import foo from "file:///path/to/mod.ts"` didn't
get inserted automatically:

```ts
/**
 * ```ts
 * console.log(foo);
 * ```
 * 
 * @module
 */

const foo = 42;
export default foo;
```

This commit fixes this issue and the above example works fine.

---

Fixes #25718

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