Timestamps not parsed inside bold and italic text

Timestamps (e.g. <t:1708551317>) are correctly parsed when wrapped in strikethrough (~~), but are silently treated as plain text when wrapped in bold (**) or italic (_).

Steps to reproduce:
Send the following messages in Rocket.Chat:

1. ~<t:1708551317>~ → ✅ renders as formatted timestamp inside strikethrough
2. **<t:1708551317>** → ❌ renders as raw text <t:1708551317> inside bold
3. _<t:1708551317>_ → ❌ renders as raw text <t:1708551317> inside italic

Expected behavior:
Timestamps should be parsed and rendered correctly inside bold and italic text, just like they are inside strikethrough.

Root cause:
In grammar.pegjs, StrikethroughContent includes TimestampRules, but BoldContentPreferentialItemPattern and ItalicContentPreferentialItemPattern do not. Adding TimestampRules to both would fix this.

Start in grammar.pegjs by comparing StrikethroughContent with BoldContentPreferentialItemPattern and ItalicContentPreferentialItemPattern. Add the missing timestamp handling described in the issue, then reproduce the three messages to verify timestamps render correctly inside strikethrough, bold, and italic text.