addFirstPropertySource method
Adds the given propertySource
to the start of the composite chain.
This means it will be consulted before all previously added sources.
Useful when overriding existing property values with higher precedence.
Implementation
void addFirstPropertySource(PropertySource propertySource) {
List<PropertySource> existing = ArrayList.from(_propertySources);
_propertySources.clear();
_propertySources.add(propertySource);
_propertySources.addAll(existing);
}