static @NonNull CaptionVariable of(final @NonNull String key, final @NonNull String value) {
        return CaptionVariableImpl.of(key, value);
    }
Creates a new caption variable instance.

@param key   the key
@param value the value that replaces the placeholder
@return the variable instance
cloud-core/src/main/java/org/incendo/cloud/caption/CaptionVariable.java