Fixes formatter with sigils like ~H

[JesseHerrick]

Continuation of #37 #36. I can't push to that branch, so adding the needed changes here.

• Add sigil formatter support to the persistent BEAM formatter by wiring plugin-advertised sigils into Code.format_string!/2.
• Keep whole-file plugin formatting scoped to plugins that explicitly match the file extension, while .ex / .exs files use Elixir formatting with sigil callbacks.
• Add HEEX-focused regression coverage for a Phoenix.LiveView.HTMLFormatter-compatible plugin formatting both .heex files and ~H sigils inside Elixir files.

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Note

Medium Risk
Updates the BEAM-side formatting flow to mix plugin sigil callbacks with core Elixir formatting and to scope whole-file plugin formatting by extension, which could subtly change formatting behavior across projects relying on plugins.

Overview
Fixes plugin-based formatting for sigils (e.g. ~H) in the persistent BEAM formatter by wiring plugin-advertised :sigils into the options passed to Code.format_string!/2.

Refactors formatting dispatch to (1) run whole-file plugin formatting only for plugins that explicitly claim the file extension, (2) otherwise run standard Elixir formatting for .ex/.exs (with sigil callbacks), and (3) leave other file types unchanged. Also centralizes stderr redirection during plugin/sigil formatting to prevent plugin IO from corrupting the stdout binary protocol.

Adds a regression test that builds a minimal Phoenix.LiveView.HTMLFormatter-compatible plugin and verifies it formats both .heex files and ~H sigils inside Elixir source.

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