Update version in package.json and unit test it

[TobiasBg]

While working on #26073, I noticed that package.json's version is still at 3.7.0.
I'm not sure about the severity (as package.json is not part of releases), but Composer users might appreciate an update.

I'll attach a patch to update the version, and another one that adds a unit test to remind us of updating the version in the future, similar to the existing unit test for the version in readme.html.

[TobiasBg]

Update WP version in package.json

[TobiasBg]

Unit test to keep package.json's version up-to-date

[TobiasBg]

For reference: The unit test for readme.html was added in [26166].

[nacin]

In 26939:

Update versions in the 3.8 branch. see #26629.

[nacin]

In 26940:

Update package.json. props TobiasBg. fixes #26629.

[nacin]

While this had a commit against 3.8, I'm going to move it off the report.