fix: use correct Headers type

[Methuselah96]

Fixes

The RequestOpts interface was incorrectly using the Headers type from the DOM lib. This produces an error for TypeScript users if they have skipLibCheck set to false and are not including the DOM TypeScript lib:

node_modules/twilio/lib/base/BaseTwilio.d.ts:22:19 - error TS2304: Cannot find name 'Headers'.

22         headers?: Headers;
                     ~~~~~~~

Found 1 error in node_modules/twilio/lib/base/BaseTwilio.d.ts:22

This PR also updates the lib in tsconfig.json to match the target. This makes it so that the DOM TypeScript lib is not included, since by default the DOM lib is included unless specified otherwise. I verified that this change would have caught this issue.

Checklist

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

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