Fix type declarations

[hpello]

Description

The current types in index.d.ts file are not valid typescript. Namely, they contain the following errors:

index.d.ts:1:1 - error TS1046: Top-level declarations in .d.ts files must start with either a 'declare' or 'export' modifier.
1 namespace Receptacle {
  ~~~~~~~~~
index.d.ts:43:76 - error TS2707: Generic type 'Receptacle<T, X>' requires between 1 and 2 type arguments.
43     public set(key: string, value: T, options?: Receptacle.SetOptions<X>): Receptacle;
                                                                              ~~~~~~~~~~
index.d.ts:45:32 - error TS2371: A parameter initializer is only allowed in a function or constructor implementation.
45     public expire(key: string, ms: number = 0): void;
                                  ~~~~~~~~~~~~~~

Contents

In this PR:

• We fix the current typing errors
• We add a type-check target that ensures types are well checked for the future

[hpello]

Hello @DylanPiercey! Could you take a look at this PR? 🙂
We need it to keep using receptacle in our TS codebase.

[hpello]

Hi! @DylanPiercey did you have a chance to look at these typings please? 🙏
cc @sod @gamtiq