Make token pattern a dropdown list with commonly used patterns

[qetza]

Replace the token prefix and suffix parameters with a token pattern parameter providing commonly used patterns and a 'custom' option to allow to define custom prefix and suffix:

• #{MyVar}# (default to minimize breaking changes)
• #{MyVar}
• __MyVar__
• {{MyVar}}
• custom

This will be a breaking change for users who have customized their pattern.

[qetza]

The new task version 4.x now supports a dropdown list for commonly used patterns.