Add font-size check for classes

[alexdln]

In a recent issue (#1116), we removed all hardcoded font-size classes, migrated them to our own inner classes, and agreed to use rem everywhere.

We also added an rtl:check step, which checks whether classes comply with rtl patterns. Based on this, I'm adding a check for hardcoded font-size classes and slightly refining the solution to prevent this logic from degrading