[New component]: Email template

A component in Vanilla needs to be flexible enough that it can be used across a wide range of scenarios and shouldn't be constrained to a specific number of applications.

## Visual

- very much in progress, but potentially a lot of teams would use. https://www.figma.com/design/LmBylcJRbtdIvPe7rIo4M2/24.10-Canonical-Email-templates?node-id=0-1&t=YZUlTP4jo6DT7wGn-1

## Context

- Apps that generate emails (Landscape, Juju, internal products such as promotions tool)

## State

- None

## Progressive enhancement

- designed for monitor and mobile

Start by reviewing the linked Figma design and the listed email-generating applications to establish the intended component scope. The issue names no files, tests, implementation entry point, or acceptance criteria, so the finished state cannot be determined from the current description.

Written by the indexing model from the issue text.