fix: `AcanthisMap.merge` and `AcanthisMap.extend` do the exact same thing

[dickermoshe]

One of them these should be removed:

acanthis/lib/src/types/map.dart

Lines 322 to 333 in f15aa86

	/// Add field(s) to the map
	AcanthisMap<V> extend(Map<String, AcanthisType> fields) {
	_fields.addAll(fields);
	return this;
	}
	/// Merge field(s) to the map
	/// if a field already exists, it will be overwritten
	AcanthisMap<V> merge(Map<String, AcanthisType> fields) {
	_fields = {..._fields, ...fields};
	return this;
	}

Map.addAll also overwrites existing keys.
https://api.flutter.dev/flutter/dart-core/Map/addAll.html

Sorry for not filling out the template.

[francescovallone]

Thank you for finding it. Instead of removing one of them I would prefer to fix the extend method providing the following implementation:

AcanthisMap<V> extend(Map<String, AcanthisType> fields) { 
     for(final entry in fields.entries) {
       _field.putIfAbsent(entry.key, () => entry.value);
     }
     return this; 
} 

What do you think about it?