Sanity check for non-existant files

[Richienb]

Check that the files declared in the package.json are actually there before publishing.

Description

Files specified in the [main]?, types, and exports fields in the package.json should be checked for before pushing to npm.

Is the feature request related to a problem?

Both p-queue@^7.3.1 and p-queue@^7.3.2 don't contain compiled code because the build script didn't generate them and np didn't catch it.

Possible implementation

Run the check directly after the lifecycle scripts and directly before pushing to npm.

Alternatives

Manually checking is error-prone

[tommy-mitchell]

Related: #82.