Add full TypeScript support

[larrybotha]

• [x] add svelte-checker
• [x] add svelte-preprocess
• [ ] update components to use TypeScript
  • [ ] compile Svelte components on publish to prevent users from having to compile when importing svelte-forms-lib
  • [ ] use sveld or similar to generate component definitions
• [ ] replace .js files with typed .ts files
• [ ] remove lib/index.d.ts
• [ ] generate index.d.ts from .ts files
• [ ] add tsc test to CI

[larrybotha]

NOTE: reverted TS syntax in components in #112. Without svelte-forms-lib compiling TS components to regular Svelte components, users importing svelte-forms-lib would be responsible for compilation, which is a poor UX.

Added notes to description for svelte-forms-lib taking responsibility for compiling TS components.

EDIT: See https://github.com/pyoner/svelte-form/blob/master/packages/lib/package.json for tips

[larrybotha]

NOTE: see https://github.com/mattjennings/svelte-typescript-component-template for compiling TS components and building definitions