feat: allow specifying a translation style guide

[rexxars]

Note: Work in progress, need some backend changes.

Adds the ability for users to specify a style guide. I'll let the readme do the talking:

In some cases you might want/need the translator to follow a certain style guide - for instance you might tell it not to translate certain words, or be more formal or casual. To configure this you can pass a styleguide property under the translation configuration:

assist({
  translate: {
    styleguide: `Be extremely formal and precise. Translate as if you are Spock from Star Trek.`,
  },
})

The style guide is currently limited to 2000 characters, and the translation might get slower the longer your style guide is. If the provided string is longer than the limit, the plugin will throw upon studio startup.

Note that this is currently only available on a global level - it can not be defined per-field for now.

[snorrees]

Nice! Will test this tomorrow 👍

(Some minor nits, nothing blocking)

[snorrees]

Added a "dont touch headless" to the test studio. Looking good!