Correctly purge decimal spacing classes

[kristiankozak]

Previously, decimal spacing classes such as p-0.5 were incorrectly purged out. This PR fixes this by improving the class detection regex.

More details on the issue:

• The problem were utility classes that include a period, such as px-0.5 or gap-2.5
• Such classes were correctly extracted from ERB, but incorrectly parsed out from the tailwind CSS file
• Because of the leading backslash required in CSS, e.g. the .px-1\.5 class was extracted as px-1 and it's fate was then decided by the purger as such :-)

Not sure if there was any particular reason for handling the period in the previous form of the regex, but this simple change has been working well for me so far.

Thanks for building such an awesome library, removing Webpack was a pleasure!