markdown: # + space for H1 too?

[maximebj]

In prior versions of WordPress, Heading blocks only showed H2, H3, H4 but it changed in WP 5.5

In markdown, it's impossible to fire a H1 with # + Space. It only works with 2 "#'.
Should we had # + space to get an H1 too?

Maybe there was a reason to not implement it. But I guess it was to limite H1 use in document.
More and more themes add the feature to hide the page title, so we can use our own H1 wherever we want.

[getdave]

I tested this on WP 5.5.1 and was able to replicate. All the heading levels work except #1. To replicate:

• Use a site with WP 5.5.X
• Create a new Post.
• ✅ In the main post body content type ## Hello - you will see this converted into a heading block with h2 level.
• ✅ In the main post body content type ### Hello - you will see this converted into a heading block with h3 level.
• ❌ In the main post body content type # Hello - you will see this is not converted into a heading block with h1 level. Instead the raw text remains.

Probably needs adjusting here

gutenberg/packages/block-library/src/heading/transforms.js

Lines 60 to 69 in 8ba0a42

	...[ 2, 3, 4, 5, 6 ].map( ( level ) => ( {
	type: 'prefix',
	prefix: Array( level + 1 ).join( '#' ),
	transform( content ) {
	return createBlock( name, {
	level,
	content,
	} );
	},
	} ) ),

[aristath]

Created a PR for this in #25075