Further improve natural sorting of classes

[RobinMalfait]

This PR is a small continuation of #13507 where @rvanvelzen pointed out that we are checking a character that we already compared.

Additionally, this PR also prevents re-computing the charCode twice.

Performance wise, this is very minimal, running this on the tailwindcss.com codebase I get these results:

Before:

[3.19ms] Sort
[3.06ms] Sort
[3.13ms] Sort
[3.38ms] Sort
[3.11ms] Sort
[3.05ms] Sort
[3.03ms] Sort
[3.02ms] Sort
[2.96ms] Sort
[3.18ms] Sort
-------------
[3.171ms] avg

After:

[3.24ms] Sort
[3.02ms] Sort
[3.14ms] Sort
[3.05ms] Sort
[3.17ms] Sort
[3.09ms] Sort
[3.02ms] Sort
[3.12ms] Sort
[2.94ms] Sort
[2.97ms] Sort
-------------
[3.076ms] avg