[gestalt-datepicker] Add type definitions (#47570)

* [gestalt-datepicker] Add type definitions

* Change export to default to match

* update project url

types/gestalt-datepicker/gestalt-datepicker-tests.tsx

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+import * as React from 'react';
+import DatePicker from 'gestalt-datepicker';
+
+const handleChange = (value: Date) => value;
+
+<DatePicker id="example-basic" label="Select a date" onChange={({ value }) => handleChange(value)} />;
+<DatePicker
+    disabled
+    id="example-preselected value"
+    minDate={new Date(1984, 6, 6)}
+    maxDate={new Date(2020, 6, 10)}
+    label="Alberto's birth date"
+    onChange={({ value }) => handleChange(value)}
+    value={new Date(1985, 6, 4)}
+/>;

types/gestalt-datepicker/index.d.ts

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+// Type definitions for gestalt-datepicker 13.4
+// Project: https://github.com/pinterest/gestalt/tree/master/packages/gestalt-datepicker, https://gestalt.netlify.app/DatePicker
+// Definitions by: cgu <https://github.com/czgu>
+// Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
+// Minimum TypeScript Version: 3.4
+
+import * as React from 'react';
+import { Locale } from 'date-fns';
+
+/*
+DatePicker Props Interface
+https://gestalt.netlify.app/DatePicker
+https://github.com/pinterest/gestalt/blob/master/packages/gestalt-datepicker/src/DatePicker.js
+*/
+
+export interface DatePickerProps {
+    id: string;
+    onChange: (args: { event: React.SyntheticEvent<HTMLInputElement>; value: Date }) => void;
+    disabled?: boolean;
+    errorMessage?: string;
+    excludeDates?: ReadonlyArray<Date>;
+    helperText?: string;
+    idealDirection?: 'up' | 'right' | 'down' | 'left';
+    includeDates?: ReadonlyArray<Date>;
+    label?: string;
+    localeData?: Locale;
+    maxDate?: Date;
+    minDate?: Date;
+    nextRef?: React.Ref<any>;
+
+    placeholder?: string;
+    rangeEndDate?: Date;
+    rangeSelector?: 'start' | 'end';
+    rangeStartDate?: Date;
+    ref?: React.Ref<any>;
+    value?: Date;
+}
+
+export default class DatePicker extends React.Component<DatePickerProps, any> {}

types/gestalt-datepicker/package.json

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+{
+    "private": true,
+    "dependencies": {
+        "date-fns": "^2.0.1"
+    }
+}

types/gestalt-datepicker/tsconfig.json

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+{
+    "compilerOptions": {
+        "module": "commonjs",
+        "lib": [
+            "es6"
+        ],
+        "noImplicitAny": true,
+        "noImplicitThis": true,
+        "strictFunctionTypes": true,
+        "strictNullChecks": true,
+        "baseUrl": "../",
+        "typeRoots": [
+            "../"
+        ],
+        "jsx": "react",
+        "types": [],
+        "noEmit": true,
+        "forceConsistentCasingInFileNames": true
+    },
+    "files": [
+        "index.d.ts",
+        "gestalt-datepicker-tests.tsx"
+    ]
+}

types/gestalt-datepicker/tslint.json

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+{ "extends": "dtslint/dt.json" }