Declare debug as optional using peerDependenciesMeta

[ylemkimon]

Related: #135 and #140.

This will result in:

• no change in package managers that don't support peerDependenciesMeta EDIT (Aug. 16): without peerDependencies declaration
• no warning for undeclared peer dependency, especially in Yarn 2 with PnP

so it's completely backward compatible.

[coveralls]

Coverage remained the same at 100.0% when pulling a6068cd on ylemkimon:patch-1 into b2f2ea5 on follow-redirects:master.

[RubenVerborgh]

Do we have any evidence that peerDependenciesMeta without peerDependencies works?
Because a version number is currently missing, so I don't think this can work.

[ylemkimon]

@RubenVerborgh

• npm doesn't support peerDependenciesMeta without peerDependencies yet, so no change in npm (https://git.io/JJdys)
• Makes peerDependenciesMeta imply peerDependencies yarnpkg/berry#531
• feat: peerDependenciesMeta implies peerDependencies pnpm/pnpm#2143

[RubenVerborgh]

What evidence do we have that this PR works?

[ylemkimon]

@RubenVerborgh I've setup a test repo at ylemkimon/peer-deps-meta-test. master branch installs the current version v1.13.0 and patch branch installs this PR. It is installed successfully with all 11 package managers without new warning. Note the warning is printed in master branch with Yarn 2 but not in patch branch with Yarn 2.

[RubenVerborgh]

Excellent, thanks!