Improve error handling for initialContent in BlockNoteEditor

[marwan37]

Description

The current implementation of BlockNoteEditor does not provide clear error messages when invalid initialContent is passed. This PR improves the error handling by adding more descriptive error messages and checks for required keys in block objects.

fixes: #342

Changes

• Enhanced error messages in BlockNoteEditor.ts for invalid initialContent.
• Introduced defaultInitialContent; initialContent defaults to this if checks fail.
• Changed initialContent from const to let for reassignment (further guidance is appreciated if undesirable).

Testing

1. When passing a string, e.g. initialContent = "abc":

2. When passing an array with non-object elements, e.g. initialContent = ["abc"]:

3. When passing an array of objects but keys are invalid, e.g. [{ someKey: "abc", id: "123123" }]:

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