fix: Separated type checks from extension building [#1441]

This MR contains the following changes:

• Removed --type-checking build argument in favor of running tsc --noEmit.
  • Merged npm run lint:ts-only and npm run test:types into npm run $ lint.ts, which is executed as part of npm run lint.
  • Updated documentation accordingly.
• Updated TypeScript configuration to only check files in the src/ directory.
  Note that this means that TypeScript files in the js/api/ directory aren't checked directly, but only indirectly via imports. We could also include js/**/*.ts in the configuration, if we want to make this more explicit, but this would become unnecessary as soon as we move those files over to the src/ directory.

This MR also contains the following unrelated changes:

• Cleaned up TypeScript-specific npm scripts.
  • Removed redundant npm run prettier:ts-only in favor of npm run prettier.
  • Merged npm run test:unit:printCoverageLink into npm run $ test.unit, which is executed as part of npm test.
  • Merged npm run test:unit:ts-only into npm run $ test.unit, which is executed as part of npm test.
• Cleaned up TypeScript configuration.
  • Rearranged properties alphabetically to make it easier to read.
  • Removed unused "outDir" property, since we're not directly using tsc for generating JavaScript files, but only via webpack.