Koenig - Convert <br>s to soft-break atoms on paste

refs TryGhost/Ghost#9623
- adds `parserPlugins` option with array of parser plugins that read node values and convert them when pasting
- converts `<br>` to a soft-break atom for line breaks
- removes leading newlines from text nodes to avoid leading spaces in the render output (common when pasting MD with line breaks)

lib/koenig-editor/addon/components/koenig-editor.js

@@ -12,6 +12,7 @@ import defaultAtoms from '../options/atoms';
 import defaultCards from '../options/cards';
 import formatMarkdown from 'ghost-admin/utils/format-markdown';
 import layout from '../templates/components/koenig-editor';
+import parserPlugins from '../options/parser-plugins';
 import registerKeyCommands from '../options/key-commands';
 import registerTextExpansions from '../options/text-expansions';
 import validator from 'npm:validator';
@@ -225,6 +226,7 @@ export default Component.extend({
         editorOptions.mobiledoc = mobiledoc;
         editorOptions.showLinkTooltips = false;
         editorOptions.undoDepth = UNDO_DEPTH;
+        editorOptions.parserPlugins = parserPlugins;
 
         let componentHooks = {
             // triggered when a card section is added to the mobiledoc

lib/koenig-editor/addon/options/parser-plugins.js

@@ -0,0 +1,27 @@
+// mobiledoc by default ignores <BR> tags but we have a custom SoftReturn atom
+export function brToSoftBreakAtom(node, builder, {addMarkerable, nodeFinished}) {
+    if (node.nodeType !== 1 || node.tagName !== 'BR') {
+        return;
+    }
+
+    let softReturn = builder.createAtom('soft-return');
+    addMarkerable(softReturn);
+
+    nodeFinished();
+}
+
+// leading newlines in text nodes will add a space to the beginning of the text
+// which doesn't render correctly if we're replacing <br> with SoftReturn atoms
+// after parsing text as markdown to html
+export function removeLeadingNewline(node) {
+    if (node.nodeType !== 3 || node.nodeName !== '#text') {
+        return;
+    }
+
+    node.nodeValue = node.nodeValue.replace(/^\n/, '');
+}
+
+export default [
+    brToSoftBreakAtom,
+    removeLeadingNewline,
+];