Rename Specificity->compare() to compareTo()

Update for #11404 as proposed in the comments
minor #11416 [CssSelector] Rename Specificity->compare() to compareTo() (barryvdh)

This PR was merged into the 2.6-dev branch.

Discussion
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[CssSelector] Rename Specificity->compare() to compareTo()

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11404
| License       | MIT
| Doc PR        | -

Update for #11404 as proposed in the comments. `compareTo` was suggested as a better name then `compare`.

Commits
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98873b8 Rename Specificity->compare() to compareTo()