Code Quality: Define enum for block types.

[itsWindows11]

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Which Operating System are you using?

Windows

Which version of data-river are you using?

0.0.1

Describe your issue.

Currently, block types are worked with as plain strings as seen here:

https://github.com/softflow24/data-river/blob/development/packages%2Feditor%2Fsrc%2Fservices%2FReactFlowMapperService.ts#L22-L34

To make things easier, we can add an enum type we can use for comparison.

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