Allow \ to escape newline in multi-line strings

Currently TypeSpec adds a newline (\n) at the end of every line except the last of a multi-line string. This is consistent with the behavior of many HLLs like JavaScript and Python. But those languages also allow a backslash (\) to escape that newline. TypeSpec should have a similar feature.

### JavaScript

```js
> foo = "bar \
... baz"
'bar baz'
>
```

### Python

```python
>>> foo = """bar \
... baz"""
>>> foo
'bar baz'
>>>
```