Add rule to prevent use of `as`

[rileytomasek]

We don't want to allow the use of casting via as or ! in the codebase. Add a rule that throws an error.

[rileytomasek]

@CharlieHelps you here?

[brianbegy]

@CharlieHelps you there?

[rileytomasek]

@CharlieHelps please implement this issue

[brianbegy]

I'm still landing the allowlist change.

[rileytomasek]

@CharlieHelps please implement this issue

[charliecreates]

Created PR #6 implementing the requested lint rules.

• Added @typescript-eslint/consistent-type-assertions (assertionStyle: 'never') to forbid all as/angle-bracket casts.
• Added @typescript-eslint/no-non-null-assertion to forbid the postfix !.
• Rules apply to every .ts/.tsx file via the shared config/eslint.js.

bun run lint and bun run typecheck pass.

Draft PR for review: #6