Moved from https://github.com/sourcegraph/cody/pull/1466

refactor: add default prompt mixin to transcripts by default 

- Remove initialization of default prompt mixin by clients.
- Update Transcript to add the default prompt mixin to all interactions
on creation.
- The PromptMixin class allows mixins to be defined that prepend text to
prompts.
- Transcript mixes in all registered PromptMixin instances into the
human prompt text before sending to the assistant.
- This centralizes default prompt handling in Transcript. Clients no
longer need to add the default prompt mixin themselves.
- This also adds the default prompt mixin to web and other clients that
previously have not initialized the PromptMixin at activation.

## Test plan

<!-- Required. See
https://docs.sourcegraph.com/dev/background-information/testing_principles.
-->

You should see `(Reply as Cody, a coding assistant developed by
Sourcegraph...)` prepended to all users' questions:


![image](https://github.com/sourcegraph/cody/assets/68532117/25d31ffa-785d-4233-a778-31ca992a104e)

---------

Co-authored-by: Rafał Gajdulewicz <rafax@users.noreply.github.com>

…ch don't support inlay models. (#1490)

ImaginaryEditor does not support InlayModel and throws an
UnsupportedOperationException; IntentionPreviewEditor throws an
IntentionPreviewUnsupportedOperationException:
UnsupportedOperationException, etc.

Fixes #1488, #1486, #1485, #1480, #1466, #1462, #1435, #1363

## Test plan

```
./gradlew test --tests InlayModelUtilTest
```

Ran the IDE and clicked between editors, requesting autocompletes.