feat: simpler validation

[cyrilletuzi]

• Added predefined constants for basic JSON schemas to simplify validation:
  • SCHEMA_BOOLEAN (shortcut for: { type: 'boolean' })
  • SCHEMA_INTEGER (shortcut for: { type: 'integer' })
  • SCHEMA_NUMBER (shortcut for: { type: 'number' })
  • SCHEMA_STRING (shortcut for: { type: 'string' })
  • SCHEMA_ARRAY_OF_BOOLEANS (shortcut for: { items: { type: 'boolean' } })
  • SCHEMA_ARRAY_OF_INTEGER (shortcut for: { items: { type: 'integer' } })
  • SCHEMA_ARRAY_OF_NUMBER (shortcut for: { items: { type: 'number' } })
  • SCHEMA_ARRAY_OF_STRINGS (shortcut for: { items: { type: 'string' } })

This is especially useful for advanced types (arrays, objects, nested types),
to avoid TypeScript limitations.

• For the same types as above, the result type is now automatically infered,
  meaning you don't need to cast anymore!

Before v7.5:

this.localStorage.getItem('test', { schema: { type: 'string' } })
.subscribe((result) => {
  result; // type: any :(
});

this.localStorage.getItem<string>('test', { schema: { type: 'string' } })
.subscribe((result) => {
  result; // type: string :)
});

From v7.5:

import { SCHEMA_STRING } from '@ngx-pwa/local-storage';

this.localStorage.getItem('test', { schema: SCHEMA_STRING })
.subscribe((result) => {
  result; // type: string :D
});

• Added better interfaces for some JSON schemas:
  • JSONSchemaArrayOf<T>
  • JSONSchemaConstOf<T>
  • JSONSchemaEnumOf<T>

Why?

• simplify validation for everyone (no more casting in most cases)
• ease migration path to v7
• makes the TS 3.2 issue (see Issue with v7 & TypeScript 3.2 for array or object validation #64) disappear for most cases (except for objects or nested types)

See documentation included in the PR.

[cyrilletuzi]

Will wait a little for #72 to anticipate as much as possible and minimize possible breaking changes in v8.

[cyrilletuzi]

Closing in favor of #74