[Gekidou] commonmark feature parity (#6303)

* Initial attempt at search highlighting for Markdown

* Remove fonts from highlighted text so that it can be bolded

* Add initial task list support

* Render markdown checkbox

* Switch to Commonmark fork

* Use searchPatterns prop

* searchPatterns prop at post level

* Update app/components/markdown/transform.ts

Co-authored-by: Jason Frerich <jason.frerich@mattermost.com>

Co-authored-by: Harrison Healey <harrisonmhealey@gmail.com>
Co-authored-by: Jason Frerich <jason.frerich@mattermost.com>

types/global/markdown.ts

@@ -3,7 +3,12 @@
 
 import {TextStyle, ViewStyle} from 'react-native';
 
-export type UserMentionKey= {
+export type SearchPattern = {
+    pattern: RegExp;
+    term: string;
+};
+
+export type UserMentionKey = {
     key: string;
     caseSensitive?: boolean;
 };