refactor: extract lang tag into an internal component

[metonym]

Closes #261

Currently, lang tag logic is duplicated in three different files. This extracts them into an internal component to reduce duplication.

It also:

• sets "initial" as the default value for CSS variables in the langtag::after rule (hence the updated snapshot test)
• adds a type check for HighlightAuto
• reuses Props/Slots/Events as much as possible

[render[bot]]

Your Render PR Server URL is https://svelte-highlight-pr-262.onrender.com.

Follow its progress at https://dashboard.render.com/static/srv-cf2sk514rebf7jh4ddeg.

[render[bot]]

Your Render PR Server URL is https://svelte-highlight-pr-262-t6pu.onrender.com.

Follow its progress at https://dashboard.render.com/static/srv-cf2sk614rebf7jh4ddi0.