Fix compilation issue with vite

[rlconst]

Without explicit type export I see an error

node_modules/@blockly/continuous-toolbox/src/index.ts (14:26): "LabelFlyoutItem" is not exported by "node_modules/@blockly/continuous-toolbox/src/ContinuousFlyout.ts", imported by "node_modules/@blockly/continuous-toolbox/src/index.ts".

Description

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[rlconst]

I've signed CLA

[cpcallen]

This change shouldn't be necessary (get better build tooling!) but it doesn't have any serious downside unless we change LabelFlyoutItem from a pure type to a hybrid type/value (e.g. a class or enum) and forget to remove the type keyword here.