fix: inc prerelease with numeric preid

[ntkme]

The semver spec says prerelease can be numeric:

<pre-release identifier> ::= <alphanumeric identifier>
                           | <numeric identifier>

This PR fixes inc prerelease with numeric preid.

E.g. inc('1.0.0-1.0', 'prerelease', '1') should return 1.0.0-1.1, but today it returns 1.0.0-1.0.

The bug is that currently it compares preid with this.prerelease[0] === identifier, and a string '1' is not equal to number 1, thus it's returning incorrect result. Change this comparison to use compareIdentifiers just like everywhere else fixes the problem.

• Fixes [BUG] npm version prerelease does not work with numeric --preid cli#3181

[coveralls]

Coverage remained the same at 100.0% when pulling 9f91452 on ntkme:fix-prerelease-with-numeric-preid into e79ac3a on npm:master.

[ntkme]

@lukekarrys Sorry for pinging but this PR is a one line bug fix that has been sitting for nearly one year without getting any attention, even though it has been confirmed as a bug in npm/cli#3181. Can you please take a look? Thanks.