Fix extracting utilities from slim/pug templates

[ledermann]

This pull request addresses an issue with extracting utility classes from slim/pug templates. Specifically, it corrects the handling of segments starting with a number to avoid the loss of relevant class names.

This fixes a regression (described in #14005) of yesterday's release v3.4.5.

It uses Number() instead of parseInt() to check whether a string is a number. This is better because:

const segment = "2xl:bg-red-500"
parseInt(segment) => 2
Number(segment) => NaN

I've added a test to ensure my change works as expected.

[thecrypticace]

I'll get a release out today. Thanks!