Linter: Implement actionview-no-unused-strict-locals rule

[marcoroth]

This pull request implements the actionview-no-unused-strict-locals rule, which reports locals declared in a <%# locals: (...) %> declaration that the partial never references.

A strict locals declaration is the contract of a partial. Every name in it tells callers "this partial needs this value", and Rails raises ActionView::StrictLocalsError when a caller passes anything that is not declared. A name the template never reads makes that contract lie: callers keep computing and passing a value that is thrown away, and readers have to search the whole template before they can be sure it is unused. These usually appear when a partial gets refactored, the markup that used the value moves elsewhere, and the declaration stays behind.

<%# locals: (name:, age:) %>

<%= name %>

Strict local `age` is never used in this partial. Callers have to pass `age:` for a value the
template never renders. Remove it from the `locals:` declaration and from the call sites.

The message varies with whether the local is required, so an optional local reads Callers can pass size: ... instead.

Resolves #1566

[github-actions]

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_{🌱 Grown from commit 8358a10}

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commit: 8358a10