Rollup 2 support

[cncolder]

Current Behavior

tsdx lock rollup@1.32.1 now

Desired Behavior

Upgrade to rollup 2

Suggested Solution

Release npm next tag tsdx@next

Who does this impact? Who is this for?

Some plugins (e.g. rollup-plugin-postcss) has drop support rollup 1

Describe alternatives you've considered

Additional context

[agilgur5]

Yea planning on doing this soon since #731 (comment) is causing a high vulnerability and requires Rollup v2.

Per #679 (comment), it wasn't easily possible before and rollup-plugin-postcss itself broke during the upgrade. Now enough time should have elapsed that it should be doable to upgrade it and all the related deps, but it's potentially quite breaking for tsdx.config.js users in potentially unexpected ways.
TS 3.8 private fields still have issues in a lot of places, but based on reports looks like we can update to TS 3.8+ safely so long as private fields aren't used.

[agilgur5]

Duplicate of #545