refactor: replace 'path-is-absolute' dep with node:path isAbsolute method

[carpasse]

Related discussion expressjs/discussions#256

[carpasse]

@wesleytodd path.isAbsolute method was added in Node.js 0.11.2, which means that, in theory, this commit introduces a breaking change. According to this discussion, we are expected to clearly mark such changes in our commits.

Should I mark this as a breaking change using the Conventional Commits BREAKING CHANGE convention?

[wesleytodd]

While I would like to get us to using CC, nothing is today so we are not enforcing any commit style at this time.

We refactored the v5 branch to still test in old node versions as optional checks but have already merged changes which break before node 6 IIRC. So this should not block anything.

As for this, if you could add this to the history file under unreleased changes that would be awesome. Not a big deal, but I forgot to ask for it on a few commits and so am trying to remember.