Validate emojis against JSON dataset

[dannymidnight]

Turns out all_emoji_names.txt isn't the source of truth for Buildkite emojis.

This change validates template emojis against the following files:

• https://github.com/buildkite/emojis/blob/main/img-buildkite-64.json
• https://github.com/buildkite/emojis/blob/main/img-apple-64.json