fix(seg layer): Save anchor segment in JSON state as string not BigInt if defined

[seankmartin]

Fixes #897

The issue was that the anchorSegment ID in segmentation layers was being saved into the JSON state directly as a big int if it was defined, instead of being saved as a string like other segmentation IDs. This changes the anchorSegment to be saved as string if defined.

This bug did not affect all of the application, as many parts of the application use a bigintToStringJsonReplacer when calling JSON.stringify on the JSON state. However, the state editor does not use this replacer pattern nor does the screenshot actions in src/python_integration/screenshot.ts. I have added the bigintToStringJsonReplacer call to the screenshot action to be on the safe side there. But I'm unsure if it should be added to the JSON state editor. Adding this protection to the editor could possibly help hide a case where a big int value is incorrectly stored in the state and potentially incorrectly parsed from the state.

[chrisj]

I could go either way for adding the protection but since adding it would reduce the chance of a user error, it seems like the right choice.

[seankmartin]

I could go either way for adding the protection but since adding it would reduce the chance of a user error, it seems like the right choice.

Cheers Chris, added it in so!