[#1936] Migrate store.js to TypeScript

[vvidday]

Part of #1936

Proposed commit message

Currently, despite the addition of TypeScript support, the frontend is
still largely written in JavaScript. This results in a lack of type
safety and many complex objects being passed around as unknown types,
which may in turn lead to errors.

Let's migrate the store.js file to TypeScript, to get types for the all
the states that are stored within the Vuex store. This will provide a
good starting point to migrate the Vue files that access and mutate the
state of the store.

Other information

The declaration of the module and ComponentCustomProperties in vuex.d.ts provides type definitions to the this.$store object that's used in the .vue files.

Also, although I have checked on my end, this PR should probably not be merged until after #1913 , so we can see the updated results of the CI that includes the new TypeScript rules.

[ckcherry23]

Thanks for your work! I just have a few questions about typing.

[ckcherry23]

LGTM!

[vvidday]

Just added a ESLint exception in c010d83 for the empty interface in preparation for #1913 to be merged - empty interfaces are usually not meaningful, but this case is an exception.

Because the empty/"repeated" interface is inside the declare module block, which is used to type the this.$store within Vue components. We can't export to other files from within the declare module block, hence the need to declare StoreState at the top level (for other TS files), and once more in the declare module block for the Vue-component-specific typings.

[zhoukerrr]

LGTM! @ckcherry23 caught the points that I want to bring up. Thanks for the help!

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