- added @types/babel__core, @types/babel__traverse, and
  @types/lodash.merge and removed their declarations in env.d.ts
  - also removed rollup-plugin-terser's declaration there because it
    has types built-in
  - remaining actually don't have types per in-line comments

- this improved type-checking allowed TS to do stricter checks and so
  exposed a few type errors that were fixed as well
  - @rollup/plugin-babel depends on @types/babel__core as well, so the
    declaration here had previously overridden it
    - `custom` had to be augmented as it's not an option of
      `@rollup/plugin-babel`
    - `passPerPreset` did too though... it's not in `@types/babel__core`
      but it is a (deprecated) Babel option
      - logs seemed to show it was `false` regardless of what was
        passed in though...
  - Rollup's `plugins` now started complaining that `false` wasn't a
    valid Plugin either, so added a `.filter(Boolean)` but that didn't
    fully fix it so had to type-cast

- Jared had asked in a review comment back in December to consistently
  use capital TSDX (not `tsdx`) so this establishes that throughout the
  rest of the codebase
  - I don't think any of these lines/files/variables were written by me,
    so I'm just fixing up existing ones

- this came up as I was refactoring typings, so wanted to make sure
  everything was consistent as I went

- rename variable TsdxOptions to TSDXOptions
- rename babelPluginTsdx to babelPluginTSDX
  - and rename the file to this as well
    - and links to the file in the docs
- clarify extractErrors error message to specify `tsdx build`, not just
  "tsdx"
- use TSDX instead of `tsdx` in template docs
  - don't change contributing docs though, because in that one it's
    referring to the bin global or repo `tsdx`, which is correct

- previously I had used the `[typeX, ...typeX[]]` TS syntax in types,
  which means an array of at least one of those types
  - I actually had to look up how to do this when I originally wrote it
    and it even confused Shawn when I did
  - now I've looked at a few times and it takes me a second to realize
    what that means again, so hopefully this change makes it easier to
    understand the type at-a-glance and has less mental overhead

- `RollupOptions & { output: OutputOptions }` was used a few times
  in the codebase, so create a type makes this re-usable and consistent
  - so it doesn't have to be augmented everywhere in-line like that
  - and also documents its meaning

- another PR by a contributor exposes the types for `tsdx.config.js` in
  order to be used in JSDoc, so this makes it so only a single type is
  needed and exported
  - the exact type needed is the only exported basically
  - and that we're not re-exporting Rollup types directly