fix(ts): allow undefined for optional variables

[bbugh]

This fixes #961, fixes #948, fixes #998, and closes #955. Also should fix the issue raised in dotansimha/graphql-code-generator#3679.

The problem was that I added typing in #895 with the expectation that if TVariables was specified, then variables should be required. This is not the case, since queries, subscriptions, and mutations can have variables that are all optional. This is written up in #961.

This has the positive benefit of simplifying the Frankenstein's Monster that the types in useMutation were becoming.

[beeplin]

Also get the #948 problem. Hope it solves that.

[NickBolles]

@Akryum could we get this merged? I've tested it and it seems to work well.

[seanaye]

This doesn't seem to be getting merged, so how would one install this PR? I tried yarn add vuejs/vue-apollo#962/head but this is a monorepo and it gives me all sorts of errors when I try to compile

[tobias-kuendig]

@seanaye A temporary workaround is to add skipLibCheck: true to your tsconfig.json. But be aware that this will disable all type checking for anything in node_modules.

[bbugh]

@Akryum what can we do to help get this merged? Vue 3 is rolling along but all vue-apollo TypeScript users are currently blocked from upgrading to v4 by this.