txt2tags: improvements and fixes, use the default color style

txt2tags.xml:

• Remove hard colors, use default color style. A style similar to Markdown is used.
• Fix errors in the file: avoid unnecessary context switches.
• Update highlighter based on documentation and on the official highlighter for VSCode.

Sources:

• https://txt2tags.org/markup.html
• https://txt2tags.org/sample.html
• https://github.com/txt2tags/vscode