fix: Add method overload to VoiceResponse.prototype.play

[DCtheTall]

Fixes #538.

This allows users to use a string as the first argument with TypeScript enabled.

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• I have updated my branch with the master branch
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified

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[DCtheTall]

I did not check the "added tests" box because this change only affects a type declaration file. If there is a way to add test coverage to this change, I am happy to add it to this PR 😄

[childish-sambino]

For a test, you can add a line like this to examples/typescript/example.ts: twiml.play('https://demo.twilio.com/docs/classic.mp3');

That file gets compiled as part of the unit tests.

[DCtheTall]

@childish-sambino Done.

[childish-sambino]

FYI, I've updated the generator to apply this change universally: #545