Fix types and exports of the package

Fixes #170. This fix employs several strategies to achieve compatibility with all resolution strategies (node10, node16 - cjs, node16 - esm, bundler).

Exports are split into import and require sections — required due package having a type "module" in package.json
Types of the main export (single-spa-react.d.ts) are copied during the build process — required by TypeScript (one file can not represent types of both CJS and ESM export, even if they are identical)
Additional types (index.d.cts) for parcel CJS format are added. This file is not identical to the index.d.ts (see the last export line).
For node10 resolution strategy, parcel folder containing package.json redirects is inserted into the build. This is required for tooling that does not support subexports. Such tools will look into the "parcel" folder when doing "import ... from 'single-spa-react/parcel", locate the package.json, read the "main"/"types" field and be able to locate the CJS export (such tooling only supports CJS format).

Here is the analysis by https://arethetypeswrong.github.io/. NOTE: Due to rollup not inserting a compatibility layer when transpiling, in rare instances, consumers might have to access the main export with ".default". This is caused by a mix of named, and default exports.

single-spa / single-spa-react

Fix types and exports of the package #197

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