fix: use `gpt-4o` as default model (copilot-extensions#63)

this aligns with other uses across github

feat: `create*Event` methods now return strings (copilot-extensions#59)

BREAKING CHANGE: All `create*Event` methods now return a string instead
of an object with a `.toString()` method.

Before:

```js
esponse.write(createTextEvent("Hello, world!").toString());
```

Now:

```js
esponse.write(createTextEvent("Hello, world!"));
```

feat(prompt): `prompt.stream()`, `options.endpoint` (copilot-extensio…

…ns#57)

BREAKING CHANGE: `prompt()` is now throwing an error if the API responds
with an error status code

fix(prompt): `parameters.model` is now a string and is optional. It d…

…efaults to `gpt-4` (copilot-extensions#53)

- **fix(types): `ModelName` is a string now**
- **docs(README): `model` parameter is no longer required for
`prompt()`. It defaults to `gpt-4`**
- **fix: `model` parameter is now optional for `prompt()`. It defaults
to `gpt-4`**

feat: `getFunctionCalls()` -b closes (copilot-extensions#50)

feat: `options.messages` for `prompt()` (copilot-extensions#49)

feat: `options.tools ` for `prompt()` (copilot-extensions#48)

feat: `prompt()` (copilot-extensions#39)

feat: `parseRequestBody()`, `transformPayloadForOpenAICompatibility()…

…`, `verifyAndParseRequest()`, `getUserMessage()`, `getUserConfirmation()` (copilot-extensions#34)

feat: `createAckEvent()`, `createTextEvent()`, `createConfirmationEve…

…nt()`, `createReferencesEvent()`, `createErrorsEvent()`, `createDoneEvent()` (copilot-extensions#32)