create a validation suite that makes it possible to check if strings use the correct syntax

[ivanhofer]

Useful for the importer feature, a validation of all translations could be performed to detect errors before they are written to disk.

Services like inlang could validate translations and provide an instant feedback to users before they can hit 'save'.

Also as a CI process everything could be validated, since currently it is not possible to make translations 100% compatible with the base-translation.

It should be possible to validate:

• single strings if they use the correct typesafe-i18n syntax for variables etc.
• a dictionary, if everything is a string or a nested dictionary
• translations against a base-translation to see if every key is present and if all variables from the base-translation are also present in the translation