Markdonw block: fixes issue pasting Markdown

There was a bug when pasting Markdown code containing any type of list
in the live preview. The Markdown parser, Markdown-It, removes empty
space at the beginning of new lines, making impossible to paste lists
and nested lists. Also, changes the whitespace CSS property for
paragraphs inside the Live Preview component, to `pre-wrap` so white
space and new lines are not collapsed.

modules/markdown/assets/css/jetpack-markdown-block.css

@@ -5,6 +5,7 @@
 }
 .wp-block-jetpack-markdown-block-live-preview p {
 	min-height: 1.8em;
+	white-space: pre-wrap;
 }
 .wp-block-jetpack-markdown-block__live-preview__token {
 	/* $dark-gray-300 from Gutenberg _colors.scss */

modules/markdown/assets/js/components/markdown-live-preview.jsx

@@ -16,14 +16,17 @@ const {
 	createElement
 } = window.wp.element;
 
-const markdownIt = new MarkdownIt( 'zero' ).enable( [
-	'heading',
-	'emphasis',
-	'backticks',
-] );
+const markdownIt = new MarkdownIt( 'zero' )
+	.set( { breaks: true } )
+	.enable( [
+		'heading',
+		'emphasis',
+		'backticks',
+		'newline',
+	] );
 
 /*
- * Redefines the rules applied by the parser to render each token.  This adds
+ * Redefines the rules applied by the parser to render each token. This adds
  * to each token's content, the delimiter the user used (which the parser
  * obviously removes in the resulting HTML)
  */
@@ -64,6 +67,40 @@ markdownIt.renderer.rules.heading_open = function( tokens, idx ) {
 	}
 	return `<${ token.tag }><span class="${ LIVE_PREVIEW_TOKEN_CSS_CLASS }">${ token.markup }</span>`;
 };
+// Overrides the newline rule to keep whitespace at the beginning of new lines
+// Original source from `markdown-it/lib/rules_inline/newline.js`
+markdownIt.inline.ruler.at( 'newline', function replace( state, silent ) {
+	let position = state.pos;
+
+	if ( state.src.charCodeAt( position ) !== 0x0A/* \n */ ) {
+		return false;
+	}
+
+	const lastCharPosition = state.pending.length - 1;
+
+	// '	\n' -> hardbreak
+	// Lookup in pending chars is bad practice! Don't copy to other rules!
+	// Pending string is stored in concat mode, indexed lookups will cause
+	// convertion to flat mode.
+	if ( ! silent ) {
+		if ( lastCharPosition >= 0 && state.pending.charCodeAt( lastCharPosition ) === 0x20 ) {
+			if ( lastCharPosition >= 1 && state.pending.charCodeAt( lastCharPosition - 1 ) === 0x20 ) {
+				state.pending = state.pending.replace( / +$/, '' );
+				state.push( 'hardbreak', 'br', 0 );
+			} else {
+				state.pending = state.pending.slice( 0, -1 );
+				state.push( 'softbreak', 'br', 0 );
+			}
+		} else {
+			state.push( 'softbreak', 'br', 0 );
+		}
+	}
+
+	position++;
+
+	state.pos = position;
+	return true;
+} );
 
 const renderHTML = function( source ) {
 	if ( source ) {
@@ -96,7 +133,7 @@ const triggerOnChange = function( evt, source ) {
 };
 
 const sourceIsEmpty = function( source ) {
-	return ! source || '' === source.trim();
+	return ! source;
 };
 
 const ignoreLastInput = function( source ) {