Add the optional argument `fallback_policy` to the `map_text` filter

This PR describes the addition of a new optional argument to the map_text filter to define the behavior of this filter when an entry doesn’t exist in the weaver.yaml config file.

The map_text filter already supports the following arguments: map_text("<text_map_id>" [, "<default_value>"]).

This issue adds the concept of a fallback policy, defining more precisely the behavior of this filter when the input doesn't match any entries in the text_maps configuration.

The new optional parameter fallback_policy will accept one of the three possible configurations:

• input: When this behavior is set, and there is no entry matching the input, the input is returned by the filter. This was the behavior of the filter when no default value was provided.
• default: When this behavior is set, and there is no entry matching the input, the default value is returned by the filter. This was the behavior of the filter when a default value was provided. Using fallback_policy=default and not providing a default_value is considered an error, and the filter will emit an error.
• error: When this behavior is set, and there is no entry matching the input, the filter will emit an error. This specific configuration was not supported in the previous version of the map_text filter.

By making the fallback policy explicit, it is now possible, for example, to specify that we want to fail the doc/code generation if the text_map configuration is incomplete.

Start by locating the map_text filter and the text_maps configuration handling, then inspect how its existing default argument is parsed and applied. The work is done when input, default, and error fallback policies are supported, including validation for default without a default value and the corresponding error behavior.