Fix operator precedence bug dropping custom className on iframe embeds

[jphilung]

Problem

The className concatenation for the iframe embed had a missing set of parentheses, causing a subtle operator precedence bug:

(iframeProps?.className ? `${iframeProps.className} ` : '') + isYouTubeEmbed
  ? 'vds-youtube'
  : 'vds-vimeo',

Since + has higher precedence than the ternary ?:, this actually evaluates as:

((iframeProps?.className ? `${iframeProps.className} ` : '') + isYouTubeEmbed)
  ? 'vds-youtube'
  : 'vds-vimeo'

Two consequences:

1. Any iframeProps.className passed in gets concatenated with the string-coerced isYouTubeEmbed value into a throwaway string, then discarded — the custom class never makes it onto the element.
2. That concatenated string is always non-empty, so it's always truthy — meaning the ternary always resolves to 'vds-youtube', regardless of the actual value of isYouTubeEmbed.