npm: updated to 6.13.1

v6.13.1:
fix(fund): support funding string shorthand
should not publish tap-snapshot folder
Add preliminary WSL support for npm and npx
print quick audit report for human output

v6.13.0:
add fund command
delete ps1 files on package removal
update supported node list to remove v6.0, v6.1, v9.0 - v9.2

v6.12.1:
add node v13 as a supported version
Fix regression in lockfile repair for sub-deps
resolve circular dependency in pack.js

v6.12.0:
Now npm ci runs prepare scripts for git dependencies, and respects the --no-optional argument. Warnings for engine mismatches are printed again. Various other fixes and cleanups.

(adam)