feat: CSS variables for placeholders

[matthewlipski]

This PR allows for setting CSS variables to customize the default placeholders. It contains only basic functionality as it's meant as a temporary solution before we fix performance issues in #503:

• Can change default placeholder which appears in the block which contains the text cursor
• Can change placeholder which is appears after pressing the add block button
• Can change placeholders for default heading and list item blocks, which are always visible and overwrite the default placeholder
• No support for custom blocks or more advanced filtering based on block

Default placeholders:

.bn-container {
  --bn-default-placeholder: "Enter text or type '/' for commands";
  --bn-filter-placeholder: "Type to filter";
  --bn-heading-placeholder: "Heading";
  --bn-list-placeholder: "List";
}

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[YousefED]

will be supserseded by #503