Allow escaping keys in templates

[LucasPickering]

Right now templates don't allow expressing all strings. For example, there's no way to write a template that's just the raw string {{user_id}}. If you write {{user_id}}, it parses as a key and will try to render the field user_id.

A couple ideas:

• Precede key with backslash: \{{user_id}}
  • This should be fairly simple to implement in the parser. I gave it a good effort but couldn't figure it out in nom. Also tried chumsky but got bored. Maybe just needs another crack
  • We would also need to make backslashes escapable, so you can precede a template key with a literal backslash (e.g. so you can render \\{{user_id}} to \\abc123, if user_id = abc123
• Inner quotes in template, e.g. {{'{{user_id}}'}} renders as {{user_id}}
  • This is what jinja does, apparently
  • Now we need to make single quotes inside the nested quotes escapable

Definitely leaning toward the first option. However it's done, Template::dangerous should be replaced with Template::escaped, which generates a template of the raw string. Additionally, we need to update the template serialization to escape parsed content. For example, if we have a parsed template that's just the raw string {{user_id}}, then it should serialize as \{{user_id}} (or however we decide to do escaping).