ref(types): Verbosify envelope types

[lobsterkatie]

This PR refactors our Envelope types, with the goal of both slightly simplifying the implementation and (primarily) moving from having values inlined to assigning them to variables, in order to make it easier to see how all of the parts fit together.

Notes:

• In addition to the envelope types, there are two new types, UserFeedback and ClientReport, representing the payloads of their respective envelope types.
• Both envelope and item headers base types are now objects containing all optional and required properties (as well as an index signature allowing them to be extended). This allows us to avoid having to perform type intersections in the item and envelope types, thus making them easier to parse.
• All formerly inlined values are split out into variables, in order to be able to use naming as a way to show exactly what's in each successive wrapper type.
• All of the base types are exported in order to allow them to be extended in other SDKs. Also exported, primarily for the purposes of type casting, are the various item and envelope types. Header types aren't exported because they're internal to items and envelopes, which are the only two units upon which outside code should be acting.

[AbhiPrasad]

🚀

[github-actions]

size-limit report

Path	Base Size (86a526a)	Current Size	Change
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.7 KB	19.7 KB	-0.01% 🔽
@sentry/browser - ES5 CDN Bundle (minified)	63.31 KB	63.31 KB	0%
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.37 KB	18.37 KB	+0.02% 🔺
@sentry/browser - ES6 CDN Bundle (minified)	56.48 KB	56.48 KB	0%
@sentry/browser - Webpack (gzipped + minified)	22.16 KB	22.16 KB	0%
@sentry/browser - Webpack (minified)	76.18 KB	76.18 KB	0%
@sentry/react - Webpack (gzipped + minified)	22.19 KB	22.19 KB	0%
@sentry/nextjs Client - Webpack (gzipped + minified)	46.33 KB	46.33 KB	0%
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	27.16 KB	27.16 KB	0%