Linter: Treat breadcrumb as a side-effecting helper in erb-no-unused-expressions

[6temes]

What

Adds breadcrumb to SIDE_EFFECT_METHODS in erb-no-unused-expressions, so <% breadcrumb :key %> calls are no longer reported as unused expressions.

Why

gretel — a widely used breadcrumbs gem — exposes breadcrumb as a view helper that is called purely for its side effect. It sets the current breadcrumb trail and is meant to be used in a silent tag:

<% breadcrumb :product, @product %>

Under the hood it just assigns the renderer and returns it:

def breadcrumb(key = nil, *args)
  @_gretel_renderer = Gretel::Renderer.new(self, key, *args)
end

This is the same shape as content_for, provide, and the turbo_* helpers already on the allowlist: a bare, receiver-less call used for a side effect, whose return value is intentionally discarded (you can't meaningfully <%= %> it — that would print a Gretel::Renderer). Today the rule flags every <% breadcrumb %>, so an app that renders breadcrumbs on most pages gets one offense per view, which in practice forces disabling the rule project-wide.

Verification

• Added a valid cases test mirroring the existing provide / Turbo tests.
• Behaviorally confirmed against the published 0.10.1 build: with breadcrumb allowlisted, <% breadcrumb :key %> passes while a genuine <% @foo %> is still reported.

I wasn't able to run the full JS test suite locally (the workspace build pulls in the native/Emscripten toolchain), so I'd appreciate CI confirming the added test.