Linter: Improve erb-unsued-block-argument offense messages

[marcoroth]

Follow up on #1958 and #1968.

Every unused block argument that isn't an each_with_index index gets the same advice today: "Remove it, or prefix it with an underscore". This pull request makes that concrete where it can be, and stops giving it where it is wrong.

Show the tag to write

When every argument of the block is unused, the parameter list can go entirely, so the message now shows the tag the author already wrote, minus the |...|:

<% pages.each do |page| %>
  <div class="stories-progress-bar"></div>
<% end %>

- Block argument `page` is never used. Remove it, or prefix it with an underscore as `_page` to show it is intentionally unused.
+ Block argument `page` is never used. Remove it and write `<% pages.each do %>`, or prefix it with an underscore as `_page` to show it is intentionally unused.

This is not limited to each, and the tag is rewritten from the source, so whatever the block is called on is kept as it is.

The rewrite is skipped, and the message falls back to the plain Remove it, when the tag spans multiple lines, when the rewritten tag would be longer than 60 characters, or when an argument the rule does not report (&block, **options) would be left behind.

Stop suggesting a removal that rebinds the other arguments

A block destructures what it is yielded based on how many parameters it declares, so removing one argument out of several is not the same edit as removing all of them:

[[1, 2], [3, 4]].each { |a, b| } # a is 1, b is 2
[[1, 2], [3, 4]].each { |a| }    # a is [1, 2]

Following the old advice here would rebind key from the key to the whole pair, and the page would start rendering [:name, "Marco"]:

<% @pairs.each do |key, value| %>
  <%= key %>
<% end %>

- Block argument `value` is never used. Remove it, or prefix it with an underscore as `_value` to show it is intentionally unused.
+ Block argument `value` is never used. Prefix it with an underscore as `_value` to show it is intentionally unused.

So removal is only offered when every reported argument is unused. When some of them are still in use, the underscore is the only correct fix and the message says just that. The same applies to the element of an each_with_index whose index is used, and to an unused argument of a destructured |(key, value)|.

The each_with_index message from #1968 is unchanged for the shape it was written for. When the element is used and only the index is not, it still suggests each instead of each_with_index.

[github-actions]

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commit: d0fe805