fix(subscriptions): d.ts file error

[ghost]

Description

This PR fixes the missing d.ts files error Could not find a declaration file for module X ... in @zilliqa-js/subscriptions package since 0.8.6 version.

The cause is that subscriptions package is missing the following dependencies:

• @types/websocket
• mitt

The dependencies are only available in development environment.
Therefore, this PR fixes the issue by adding the missing dependencies in the subscriptions package.

You can spot these errors by running tsc --target es5 --noEmit once you install and import the subscriptions package.

Motivation and Context

How has this been tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• Add tests to cover changes as needed.
• Update documentation as needed.

[codecov-commenter]

Codecov Report

Merging #415 (fd2fd16) into dev (b659114) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #415   +/-   ##
=======================================
  Coverage   82.16%   82.16%           
=======================================
  Files          43       43           
  Lines        1811     1811           
  Branches      331      331           
=======================================
  Hits         1488     1488           
  Misses        322      322           
  Partials        1        1           

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