(fix): revert #130's breaking change of tsconfig options

[agilgur5]

• not sure why this change was made in that PR as it was unrelated to
  the rest, but it broke some things related to Babel usage and maybe
  other things

  • e.g. babel macros were not included per Breaking change with tsdx 0.12.1 with Tailwind babel macro #413
  • and it used tslib instead of adding babel helper functions

• add a comment of why it seems like the target is overridden to esnext

  • though I am not sure of the original reasoning, as it has existed
    since very early versions of TSDX, see feat: Add success/failure hooks to watch #130 comments

Fixes #413 . EDIT: Also fixes #412

Whether TS should transpile to target or Babel should is a good question to raise, but for now this just reverts the breaking change.
Should probably document in the README that tsconfig target is ignored and that users should use Babel's preset-env to configure the target instead.
Should also probably add more tests around this as bugs / breaking changes like this in TSDX's build output can have ripple-effects on libraries using it (vs. the other commands don't affect output, so the impact is limited to just library authors as opposed to library users).

[agilgur5]

Another example of .d.ts missing in Ubuntu CI tests here -- see growing list in #403

[jaredpalmer]

I forgot why, but this is v important.

[agilgur5]

Found #117, #102 (#101), and #111 that are all related to this.
Also possibly #345, but not sure about that one.

Per #117, sounds like the comment I left in the code, // TS -> esnext, then leave the rest to babel-preset-env, describes the original rationale accurately.