Linter rule to enforce typed variables

**Is your feature request related to a problem? Please describe.**
Untyped variables suffer from many issues as you outline in your docs. A simple opt-in linter rule that forces all variables to be types could prevent such issues.

**Describe the solution you'd like**
An optional linter rule (either `off` or `warning` by default I suppose) that enforces the use of typed variables, e.g. `no-untyped-vars`.

The issue names no files, tests, or entry points. Start by locating the linter rule architecture and existing variable-related diagnostics, then determine how an optional `no-untyped-vars` rule would be configured as off or warning by default. Done means the rule consistently enforces typed variables when enabled and has coverage for its diagnostic behavior.

Written by the indexing model from the issue text.