Use the error prop of the Wikit TextInput component to show an inline error if the lemma length exceeds a certain maximum length. Do this using the input event, i.e. as soon as the lemma becomes too long (rather than only on submit).
The length needs to be passed from WikibaseLexeme.git into the Vue app. Currently, the limit of 1000 (Unicode code points) is hard-coded in WikibaseLexeme.entitytypes.repo.php; making it configurable is beyond the scope of this task, but we might want to at least extract a PHP constant on some related class, so that we can refer to it in the ResourceLoader module definition without copying the value directly.
A preliminary message will be added at T305854#7937644, which should only be added for the dev entry point initially (i.e. in DevMessagesRepository.ts); the final message, to be translated as well, will be decided afterwards.