Officially support developing Custom Applications in TypeScript

[emmenko]

This is an umbrella issue for the official TypeScript support for developing Custom Applications.

TODO:

• sharable tsconfig (similar to https://github.com/tsconfig/bases) (see feat(application-config): shared tsconfig.json #2628)
• update prettier config to use TS parser
• update jest config to use TS preset
• adjust linters to include TS file extensions ts, tsx
• add custom @types declarations, if needed (see https://vitejs.dev/guide/features.html#client-types as an example)
• missing type declarations for e.g. nodejs packages (sync-actions)
• graphql generated types (see [Idea] Sharing graphql code generation setup #1973)
• starter template (see TS version of the starter kit #2297)
• document TypeScript support in Custom Applications (see docs: TypeScript support in Custom Applications and release v21.8 #2624)
• provide TS version of all JS code snippets in the documentation (see [WIP] docs: provide TS version of code snippets #2638)