Check whether ISBN already exists if manually adding a book

When searching for a book or author, it is sometimes not found. Searching for an ISBN might cause a delayed import that will not be found immediately.

In that case, I feel inclined to manually import a book. Despite entering the ISBN, there is no check whether this ISBN already exists. This might cause duplicates.

### Describe the problem that you'd like solved
If I enter an ISBN in the add book page, there should be a background check on whether this ISBN already exists to prevent duplicates.

If a work already exists, I should get a notification and be asked whether those books might need to be merged or an error needs to be reported (for example, because an edition is connected to the wrong work).

### Proposal & Constraints

The author field has a similar functionality, which looks for author with similar spellings.

### Additional context

#3473
#7055

### Stakeholders
@mekarpeles @cdrini