[Ballerina OpenAPI] Default to --single-file mode when outside a Ballerina project

[SachinAkash01]

Description

The Ballerina packages are identified by a Ballerina.toml file: https://ballerina.io/learn/organize-ballerina-code/#package-structure
If it is not found, the Ballerina files are treated as individual files. So if we try to generate a service implementation outside a Ballerina project from a given OpenAPI contract, the VSCode plugin states types can't be found. In this case, we can generate the service and types as a single file as follows,

bal openapi -i openapi.yaml --mode service --single-file

This issue is created to improve this to detect if the user is in a Ballerina project and default to single file mode if not.

Version

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