Implement pnpm adapter

[Turbo87]

As discussed via Discord I've created a new dependency manager adapter for pnpm, largely based on the existing npm adapter.

The main differences to the npm adapter are:

• We do not backup the node_modules folder and instead run pnpm install once more in the cleanup phase
• We do not use --no-lockfile so that we only have to resolve the dependencies that have actually changed, resulting in a much smaller amount of necessary dependency downloads

I've implemented a usePnpm option, similar to the existing useYarn option. I'm aware that this might not be ideal, but it works well enough for now and can still be refactored later.

Resolves #744

[codecov]

Codecov Report

Merging #787 (553b5ab) into master (cb5f901) will decrease coverage by 1.10%.
The diff coverage is 88.50%.

❗ Current head 553b5ab differs from pull request most recent head c2e80da. Consider uploading reports for the commit c2e80da to get more accurate results

@@            Coverage Diff             @@
##           master     #787      +/-   ##
==========================================
- Coverage   95.46%   94.36%   -1.11%     
==========================================
  Files          16       17       +1     
  Lines         463      550      +87     
==========================================
+ Hits          442      519      +77     
- Misses         21       31      +10     

Impacted Files	Coverage Δ
lib/utils/dependency-manager-adapter-factory.js	87.50% <50.00%> (-7.50%)	⬇️
lib/dependency-manager-adapters/pnpm.js	90.36% <90.36%> (ø)

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[Turbo87]

This can be seen in action at mainmatter/ember-test-selectors#754

[kategengler]

@Turbo87 added a warning about experimental support for pnpm. When we feel comfortable truly supporting we will add documentation to the README and remove the warning.

[buschtoens]

Fantastic! Thanks a lot for shipping this. 😍