- use rpt2 as a `configPlugin` within our own build process, as well as to build itself
  - doubly dogfood

- we don't have any tests for `configPlugin`s, and it's a Rollup CLI option, so this is one of the few ways to test it
  - create a helper `_build-self` script so we don't need to type/repeat as much (or have as much potential for mistakes)

NOTE: this commit fails to build, need to make some more adjustments to the base config to get it to work in TS
- will do that in the next commits so that it's easier to read/differentiate/review

- replace the unmaintained `rollup-plugin-re` w/ `@rollup/plugin-replace`
  - `rollup-plugin-re` hasn't had a commit in ~5 years and doesn't have typings (natively or in DT)
  - also import the plugins in the same order that they're used
- `require("package.json")` -> `import`
  - and add `resolveJsonModule` to `configPlugin` `tsconfigOverride` to handle this
    - only used by the `configPlugin`, not everything else, so not added to `tsconfig.json`

- fix the usage of the `resolve` plugin as it didn't type-check/had outdated usage

- need to use `createRequire` in ESM mode
  - deps: bump `@types/node` to support the types for that as well
  - this is a BREAKING CHANGE as it requires Node 14+
- fix `fs-extra` error due to it being CJS

- add `build-self/*` to `prebuild` clean
  - not to `build-self` script, as the `build-self/` directory has to exist for that
- output as `build-self/index.mjs` instead of `build-self/dist/rollup-plugin-typescript2.es.js`
  - so that types work out of the box -- due to `index.d.ts`
  - and so that Rollup correctly loads it as ESM and not CJS

- set `engines.node` in `package.json` to show this as well