Upgrade typescript- and linting-dependencies

Attempts to fix linter parsing issues stemming from changes in typescript v3.7.

- Upgrade `eslint`-related packages.
- Upgrade `typescript` and related `@sindresorhus/tsconfig`.
- Fixes linting errors due to upgraded rules.
- Note that you might need to remove `node_modules` and run `npm install` again to properly clear linting caches.
- Include the `test` directory in a separate `tsconfig.xo.json` to fix the below parsing error.

> Parsing error: `"parserOptions.project"` has been set for `@typescript-eslint/parser`.
> The file does not match your project config: `test/test.ts`.
> The file must be included in at least one of the projects provided.

See

- https://github.com/typescript-eslint/typescript-eslint/pull/1045
- https://github.com/microsoft/TypeScript/commits/v3.7.5
- https://github.com/sindresorhus/tsconfig/issues/7
- https://github.com/xojs/eslint-config-xo-typescript/issues/20
- https://github.com/xojs/eslint-config-xo-typescript/issues/15

source/index.ts

@@ -278,8 +278,8 @@ is.typedArray = (value: unknown): value is TypedArray => {
 };
 
 export interface ArrayLike<T> {
-	readonly length: number;
 	readonly [index: number]: T;
+	readonly length: number;
 }
 
 const isValidLength = (value: unknown): value is number => is.safeInteger(value) && value >= 0;