[whatwg/dom] Proposal: Spellcheck API (#547)

**Purpose**: allow websites to check spelling according to browser rules by JavaScript API with results similar to [spellcheck](https://html.spec.whatwg.org/multipage/interaction.html#attr-spellcheck).
**Concerns**: user defined words allows fingerprinting, raising privacy issues.

**API**: 
```typescript
navigator.spellchecker(language: DOMString) => Promise.<Spellchecker>;
```

Valid implementation can always return rejected promise (eg. no in-browser dictionary).

Valid implementation can (should?) ask user to allow access to user-defined dictionary.

```typescript
class Spellchecker {
    isValidWord(word: DOMString) => Promise.<Boolean>;
    isGramaticallyValid(text: DOMString) => Promise.<Boolean>;
    validateText(text: DOMString) => Promise.<Array.<SpellcheckingRangeResult>>
}

class SpellcheckingRangeResult {
    int start; // Codepoint or character number?
    int end; // Codepoint or character number?
    enum('ortography', 'grammar', 'other') type; 
    dictionary details; // Implementation-specific details. 
}
```

`isValidWord` returns if word is valid. If user didn't allow to access user-defined dictionary, it should return promise resolving to `false`. It should return promise resolving to `null` if not-a-word is given (eg. emoji, whitespaces, whole sentence).
 
`isGramaticallyValid` checks text for presence of grammar errors. Implementations not supporting grammar-checking should return promise resolving to `true`. 

`validateText` checks text for presence of all errors. It should return Promise resolving to array of `SpellcheckingRangeResult`s. 

`SpellcheckingRangeResult` is a simple object with read-only properties `start` and `end`, coresponding to start and end of "suspicious" text fragment. `type` property describes issue with selected fragment. Implementation-specific issues (eg. hypothetical plugin checking for formal language) should set `type` to string `"other"`. `SpellcheckingRangeResult` should be used mainly to display feedback to user. 

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