Put together a more thorough CONTRIBUTING guide

[LukeAbby]

This effectively boils down to the many pitfalls of TypeScript. Common issues seems to be:

• Determining the right type for a plain object. Usually Record<string, unknown>.
• Using InexactPartial appropriately.
• Not writing foo?: T | undefined in function signatures; only for object properties does the distinction between foo?: T and foo?: T | undefined matter.
• Writing effective tests.
• The process of diffing versions.
• Getting started with the helper types.
• etc.

Ideally if lints could be find for these it'd ease the burden into becoming more automated.

[JPMeehan]

Part of #2528

[JPMeehan]

Some more

• What to do when the core type documentation is wrong
• Deprecation handling
• Explanation of fields.mjs
• Policy for protected vs internal vs private

[LukeAbby]

• Setting up VSCode with ESLint/TypeScript/other recommended extensions.
• Why we do export {} in every new file that doesn't have an import/export. (To make sure the files aren't automatically ambient)
• Running tests.
• How to run tsc to generate type definitions from Foundry source. This is technically a part of diffing versions but I listed it here to make sure I won't forget.
• A quick guide on namespaces. They're a pretty unusual thing in TypeScript these days which I actually think is a bit of a shame.
• How to find a good first issue.
• Explanation of the advanced patterns in the repo for things like Mixins
• Possibly a brief explanation of the helper types.
• What Foundry documentation to copy over. For example you probably shouldn't copy over /* ************** */ comments.
• What documentation to add. For example if something is only initialized during a certain hook.
• Common Foundry TSDoc mappings e.g. Object<T> -> Record<string, T> or so on.
• When to use MaybePromise.
• How to provide the defaults of something, e.g. a function parameter.