Linter: Implement actionview-no-redundant-local-assigns rule

[marcoroth]

This pull request implements the actionview-no-redundant-local-assigns rule, which reports local_assigns lookups in a partial that already has a <%# locals: (...) %> declaration, where the declaration makes the lookup either redundant or dead.

local_assigns stays useful in a partial that has a strict locals declaration, so this is deliberately not a rule against local_assigns itself. What it targets are the lookups the declaration already answers: a required local is a local variable, so reading it back out of the hash is a longer way to write the name, and it is always present, so asking key? about it is a condition that can only take one branch. A name the declaration does not mention can never arrive at all, since Rails raises ActionView::StrictLocalsError for callers that pass an undeclared local, which makes the lookup dead code.

<%# locals: (user:) %>

<%= local_assigns[:user].name %>

Strict local `user` is already a local variable in this partial, so `local_assigns[:user]` reads
back a value that is already in scope. Use `user` instead.

<%# locals: (user:) %>

<% if local_assigns.key?(:user) %>
  <%= user.name %>
<% end %>

Strict local `user` is required, so `local_assigns.key?(:user)` is always `true`. Remove the
condition, or give `user` a default value to make it optional.

<%# locals: (user:) %>

<%= user.name %>
<%= local_assigns.fetch(:size, "large") %>

`size` is not declared in the `locals:` declaration, so Rails raises if a caller passes it and
`local_assigns.fetch(:size, ...)` can never find it. Declare `size:` in the declaration, or
remove the lookup.

Resolves #2011

[github-actions]

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commit: 57a4a5d