Feat/custom instruction

[KartikGS]

fixes #748

Adding anchor to input custom instruction in Settings layout and adding new route for custom instruction input in src/routes/settings/custom.

Adding custom instruction to the setting store in src/lib/stores/settings.ts, src/lib/types/Settings.ts 1, src/lib/types/Settings.ts 2, src/routes/+layout.server.ts and src/routes/settings/+server.ts.

Appending custom instruction to preprompt in src/routes/+page.svelte 1 and src/routes/+page.svelte 2.

[nsarrazin]

Hi! Appreciate the work on this, I think it could be a useful feature, and seems pretty well implemented 😄

A few open questions for me:

• could we put the text area in /settings instead of having a separate route just for custom instructions? Not 100% sure about this
• Should custom instructions also run with assistants ? Or should they only run when using a model without an assistant.

Not sure, wdyt @gary149 @mishig25 ?