Linter: Implement html-no-nested-forms rule

[irinanazarova]

This pull request implements a new html-no-nested-forms rule that disallows placing one <form> element inside another <form> element, including forms rendered by Action View helpers.

<%= form_with model: @mission do |form| %>
  <%= form.submit "Update" %>
  <%= button_to "Delete", mission_path(@mission), method: :delete %>
<% end %>

`button_to` renders its own `<form>` element and cannot be nested inside another form. Move it outside of the enclosing form.

HTML does not support nested forms: browsers silently drop or re-parent the inner form, leading to broken submissions and buttons that submit the wrong form. The helper case is particularly easy to miss because the nested <form> never appears in the template. In the example above, the form rendered by button_to is dropped during parsing and clicking "Delete" submits the outer form instead, performing an update.

The rule detects literal <form> elements with a depth counter (same approach as html-no-nested-links) and form-rendering helpers via Prism (same approach as actionview-no-silent-helper). The helper list comes from the Action View helper registry via getHelpersForTag("form"), filtered to helpers the parser does not transform yet: once a form helper becomes supported, the element branch catches its synthesized <form> element and the Prism branch stops covering it automatically. button_to is included per the discussion in marcoroth#166, since it is effectively a hidden form. The offense is reported on the inner form's tag name, or on the helper call for helper-rendered forms.

Nesting composed across partials (an outer form in one template, an inner form in a rendered partial) is not detectable with per-file linting. tag.form and content_tag(:form) are not detected yet; the registry's detectStyle metadata encodes how to find these. This could be a follow-up.

Resolves marcoroth#166