chore: Rewrite hand-crafted JS files into TS files

[Hunga1]

Converts hand-crafted JS files into TS files and removes the accompanying *.d.ts declaration files.

Notable Changes:

• Updated exports/imports in hand-crafted library files and test files to ES module syntax
• Removed TS namespaces (no longer necessary with ES module imports)
• Added CA certificate bundle to https.Agent agent options used by this.axios.defaults.httpsAgent
• Disabled RequestClient.spec.js "should cache the CA after loading it for the first time" test. Current implementation doesn't allow the custom a https.Agent to be created at request execution with the current CA bundle for the Axios instance to use (ref: DI-2477)

Internal ref: DI-1418

[Hunga1]

Some of the tests will still fail for the following reasons:

• Auto-generated TwiML modules updated to use the TS version of the TwiML module is introduced in pending chore: Convert TwiML to typescript #838 (merged)
• Required TwilioResponsePayload.uri property conflicts with optional uri property in other interfaces that auto-generated types extend (ref: DI-2475)
• Auto-generated FlowRevisionInstance._solution property does not fulfill FlowRevisionContextSolution interface (ref: DI-2475)
• Auto-generated file imports and references to the Page exports should no longer rely on a "Page" namespace (chore: Updated Page module imports and references twilio-oai-generator#362) (merged)

[Hunga1]

Changes should be merged alongside the removal of the Page namespace references in the generated code (namespace no longer exists). Link to PR: #841 (merged)

[mattcole19]

LGTM!