feat: Add UnusedLiveViewAssign rule

[ftes]

Detect unused LiveView/LiveComponent assigns.

Conventions and limitations

• Helper functions should read the assigns map through a variable namedassigns or through socket.assigns. Reads like opts.foo are not inferred to mean LiveView assigns.
• Only literal writes via assign, assign_new, assign_async, allow_upload, stream, stream_async, and update are checked.
• Dynamic assign keys are not tracked. Examples include assign(socket, field, value), socket.assigns[field], and Map.get(assigns, field).
• Phoenix.Component.assigns_to_attributes/2 is not treated as a read. Prefer explicit attr declarations and attr :rest, :global for forwarded attributes.
• Some assigns are intended to be consumed by framework or layout code outside the current LiveView module. Use ignored_assigns to whitelist those keys.

[s3cur3]

Wow, this is really exciting! It surfaces 268 issues on our codebase at Jump. 👀 😄 I'd like to go through those and make sure there aren't any false positives (by the terms of the contract in the docs), but me and my team are out of the office until Tuesday next week. I'll plan to get back to you next week, though. 👍

[s3cur3]

It took me a second to understand what this was saying. Might be clearer as something like:

Suggested change

	- Helper functions should read the assigns map through a variable named
	`assigns` or through `socket.assigns`. Reads like `opts.foo` are not
	inferred to mean LiveView assigns.
	- Helper functions should read the assigns map through a variable named
	`assigns` or through `socket.assigns`. If you bind assigns to another
	name (like `%{assigns: opts} = socket`, followed by using
	`opts.my_attr`), the reads will not be recognized as being related
	to LiveView assigns.

[s3cur3]

Suggested change

	- Dynamic assign keys are not tracked. Examples include
	`assign(socket, field, value)`, `socket.assigns[field]`, and
	`Map.get(assigns, field)`.
	- Writes to dynamic assign keys are not tracked. Examples include
	`assign(socket, field, value)`, `socket.assigns[field]`, and
	`Map.get(assigns, field)`.

[s3cur3]

Suggested change

	outside the current LiveView module. Use `ignored_assigns` to whitelist
	outside the current LiveView module. Use `ignored_assigns` to allowlist