feat(gnoweb): allow custom markdown blocks inside gno-form

[moonia]

Closes #4969

Description

This PR adds support for custom Markdown content inside blocks via a new tag.

Usage

<gno-form>
    <gno-markdown>
        # FORM
        **Do not LIE**.
    </gno-markdown>
</gno-form>

Preview

TODO: fix margin

[Gno2D2]

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 33 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
gno.land/pkg/gnoweb/markdown/ext_forms.go	0.00%	33 Missing ⚠️

📢 Thoughts on this report? Let us know!

[Davphla]

I think this can be a good idea to not allows gno-form with no input elements (like the one showcase in example)