Escape markdown sequences (#2208)

* escape inline markdown character

* fix typo

* improve document around custom markdown plugin and add escape sequence utils

* recover inline escape sequences on edit

* remove escape sequences from plain text body

* use `s` for strike-through instead of del

* escape block markdown sequences

* fix remove escape sequence was not removing all slashes from plain text

* recover block sequences on edit

src/app/plugins/markdown/block/parser.ts

@@ -0,0 +1,47 @@
+import { replaceMatch } from '../internal';
+import {
+  BlockQuoteRule,
+  CodeBlockRule,
+  ESC_BLOCK_SEQ,
+  HeadingRule,
+  OrderedListRule,
+  UnorderedListRule,
+} from './rules';
+import { runBlockRule } from './runner';
+import { BlockMDParser } from './type';
+
+/**
+ * Parses block-level markdown text into HTML using defined block rules.
+ *
+ * @param text - The markdown text to be parsed.
+ * @param parseInline - Optional function to parse inline elements.
+ * @returns The parsed HTML or the original text if no block-level markdown was found.
+ */
+export const parseBlockMD: BlockMDParser = (text, parseInline) => {
+  if (text === '') return text;
+  let result: string | undefined;
+
+  if (!result) result = runBlockRule(text, CodeBlockRule, parseBlockMD, parseInline);
+  if (!result) result = runBlockRule(text, BlockQuoteRule, parseBlockMD, parseInline);
+  if (!result) result = runBlockRule(text, OrderedListRule, parseBlockMD, parseInline);
+  if (!result) result = runBlockRule(text, UnorderedListRule, parseBlockMD, parseInline);
+  if (!result) result = runBlockRule(text, HeadingRule, parseBlockMD, parseInline);
+
+  // replace \n with <br/> because want to preserve empty lines
+  if (!result) {
+    result = text
+      .split('\n')
+      .map((lineText) => {
+        const match = lineText.match(ESC_BLOCK_SEQ);
+        if (!match) {
+          return parseInline?.(lineText) ?? lineText;
+        }
+
+        const [, g1] = match;
+        return replaceMatch(lineText, match, g1, (t) => [parseInline?.(t) ?? t]).join('');
+      })
+      .join('<br/>');
+  }
+
+  return result ?? text;
+};