RegEx intent provider should support mapping synonyms

[gdaniel]

Supporting synonyms would also allow to match mapping values in a case insensitive way (see #259) We need synonyms for this because

• Matched mappings should return their reference value (so we cannot just accept the input in a case insensitive way as another mapping value, otherwise we are not returning the same mapping value)
• Case insensitive regex derived from the base value could be transparently added as a synonym