feat: set-button API for scripts to add custom control bar buttons

[tomasklaen]

Adds set-button <name> <data_json> message external scripts can send to add or update buttons whose state, look, and click action is defined by data_json.

Users can then add this button into their controls bar with button:<name> syntax.

Example:

mp.commandv('script-message-to', 'uosc', 'set-button', 'button-name', utils.format_json({
  icon = 'icon_name',
  active = true,
  badge = '0',
  tooltip = 'Button tooltip',
  command = 'command to send on click',
}))

Button data interface (pseudo types):

{
  icon: string;
  active?: boolean;
  badge?: string;
  command?: string | string[];
  tooltip?: string;
}

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closes #935