Refactor placeholders check to use format checks base

### Describe the problem

The logic in both checks is basically same, and it also shares logic for plurals since 883b13a61a45e3274f4ba0845ce48893e4d68471 (see #8382).

### Describe the solution you'd like

Reuse `BaseFormatCheck` logic in `PlaceholderCheck` class.

### Describe alternatives you've considered

Currently, the code is more or less copied.

### Screenshots

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### Additional context

_No response_