Remove `Tag::class()` and rename `Tag::replaceClass()` to `Tag::class()`

[vjik]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	✔️
Fixed issues	#124

[codecov[bot]]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage :thumbsup:

Coverage data is based on head (27eb58f) compared to base (ba08f35). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #136 +/- ## =========================================== Coverage 100.00% 100.00% + Complexity 766 765 -1 =========================================== Files 83 83 Lines 1988 1986 -2 =========================================== - Hits 1988 1986 -2 ``` | [Impacted Files](https://codecov.io/gh/yiisoft/html/pull/136?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft) | Coverage Δ | | |---|---|---| | [src/Tag/Base/Tag.php](https://codecov.io/gh/yiisoft/html/pull/136/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-c3JjL1RhZy9CYXNlL1RhZy5waHA=) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft)

:umbrella: View full report at Codecov.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.