Installs @types/aws-lambda and uses types in handler, per issue #4546

[jamestharpe]

What did you implement:

Closes #4546

How did you implement it:

I added the required configuration to include @types/aws-lambda as a dev dependency and updated handler.ts to make use of the Handler, Callback, and Context types.

How can we verify it:

Use the template and call npm install:

serverless create --template aws-nodejs-typescript && npm install

Todos:

• Write tests
• Write documentation
• Fix linting errors
• Make sure code coverage hasn't dropped
• Provide verification config / commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: NO

[HyperBrain]

LGTM

[horike37]

Thanks @jamestharpe for your first contribution 👍
I tested it locally and works fine 💯
LGTM