Support for escaping double quotes in string literal

Currently, there's no way to use double quotes inside a string literal. That makes it impossible to create a valid JSON string, for example.

```
┌─ script.air:22:78
│
22 │ (call relay (first_service "create_vault_file") ["{\"name\": \"file_share\"}"] config_filename)
│ ^ only alphanumeric, '_', and '-' characters are allowed in this position
```

Start at the AIR string-literal parsing and validation path indicated by the error in the issue body, and inspect how quoted characters are currently restricted. Done means double quotes can be escaped inside string literals so the shown JSON value is accepted without the validation error.

Written by the indexing model from the issue text.