Add offline spell-checking

src/spellChecker.ts

@@ -0,0 +1,15 @@
+export type Language = { name: string; code: string; longCode: string; }
+
+export type Suggestion = {
+    message: string
+    shortMessage: string
+    replacements: string[]
+    offset: number
+    length: number
+    typeName: string
+}
+
+export interface SpellChecker {
+    check(text: string, languages: string[]): Promise<Suggestion[]>
+    getLanguages(): Promise<Language[]>
+}