feat: Editor option to customize placeholders

[matthewlipski]

This PR adds an easier way to customize placeholders, using an editor option. The option currently has the following definition:

type BlockNoteEditorOptions = {
  /**
   * Determines if/what placeholder should be rendered for a given block.
   * @param block The block to render a placeholder for.
   * @param containsCursor Whether the text cursor is inside the block.
   * @param isFilter Whether the block was created using the add block button.
   * @return The placeholder string, or `undefined` if one should not be rendered.
   */
  placeholder: (
    block: Block<any, any, any>,
    containsCursor: boolean,
    isFilter: boolean
  ) => string | undefined;
  ...
}

This is both flexible while being simple, e.g. setting a single placeholder for everything goes like this:

const options = {
  placeholder: (
    block: Block<any, any, any>,
    containsCursor: boolean,
    isFilter: boolean
  ): string | undefined => {
    if(containsCursor) {
      return "Placeholder";
    }
    
    return undefined;
  };
  ...
}

I'm not sure if this is necessarily the best way of doing things vs e.g. using CSS variables, and also the function arguments can probably be streamlined a bit, but would be great to get feedback on this.

Closes #403
Closes #320

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[grutt]

Is there a plan to integrate this change in the near term? This is an open requirement for a project I'm working on.

[YousefED]

Is there a plan to integrate this change in the near term? This is an open requirement for a project I'm working on.

Yes, we want to make it possible to customize all strings soon.

Whats your exact use case?

[YousefED]

nice, one small question inline.

All tested well including nested and custom blocks?

[YousefED]

can't we use the PM API for this instead of TipTap API? (we're trying this as a general rule right?)