Pass :sigils to mix format plugins invoked for a sigil

[aptinio]

mix format passes the :sigils option to a plugin invoked for a file extension, but not to a plugin invoked for a sigil.

The plugin docs say format/2 receives "all the formatting options". The sigil formatter closure captures formatter_opts before :sigils is put into it, so a plugin invoked for a sigil is the one caller that does not get it; the extension path passes the options load_plugins/2 returned, which do carry it.

The consequence is that a plugin cannot dispatch a sigil nested inside the sigil it is formatting by passing on the options it received. Phoenix.LiveView.HTMLFormatter formats {...} attribute expressions by handing its own options to Code.quoted_to_algebra/2, so a custom sigil inside an attribute is formatted by its plugin in a .heex file but is left alone in an ~H sigil. Nothing about this is HEEx specific: it applies to any plugin that formats embedded Elixir code.

The included test fails before the change.

With :sigils present, a plugin sigil nested inside itself now dispatches to the plugin again. A plugin that formats its contents once recurses as deep as the source nests, which is finite, and the extension path already allows this today.

I checked this against Phoenix.LiveView.HTMLFormatter plus a custom sigil plugin: every sigil in the file now formats the same way whether it sits at the top level, inside ~H, or in a .heex file, and the result is idempotent.

[josevalim]

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