[api-extractor] fix: avoid using "default" keyword as var name

[adventure-yunfei]

Summary

Fixes #2804

Details

For case:

export declare interface Type1 {
    prop: import('foo').default;
}

Generating dts rollup result:

import { default as default_2 } from 'foo';

export declare interface Type1 {
    prop: default_2;
}

export { }

Ideally should generate import default_2 from 'foo';, but it needs to fix every AstImport type that may contain "default" import. Too complicated.

How it was tested

Tested cases in #2804.

[iclanton]

Can you run rush change to describe your fix?

[iclanton]

Looks like your build has a warning:

Warning: src/namedDefaultImport/index.ts:4:1 - (ae-missing-release-tag) "DefaultImportTypes" is exported by the package, but it is missing a release tag (@alpha, @beta, @public, or @internal)