Support importHelpers with module:preserve

[rbuckton]

When we shipped module: preserve we did not include full support for importHelpers, which injects an import for the tslib helper library. This changes our emit in the following ways when these two features are combined:

• When the source file is an .mts/.mjs file, we will emit import { __helper } from "tslib"; to the output file and reference the helper using the imported name (e.g., __helper).
• When the source file is a .cts/.cjs file, we will emit const tslib_1 = require("tslib"); to the output file and reference the helper using a qualified name (e.g., tslib_1.__helper).
• When the source file is a .ts/.js file, we will fall back to the .mts/.mjs behavior, above.

It's important to note that module: preserve does not imply a module format for .ts/.js input files regardless as to the value of "type" in package.json, so we will assume ESM for that case.

Fixes #59789

[rbuckton]

As an alternative for .ts/.js emit, we could choose to ignore importHelpers and emit the helpers inline, just as we do for non-module Scripts.