Infer when variables are "effectively const"

#2742 mentions the possibility of doing an early inference step where we look for variables that are assigned once and never mutated. We could add a very early check pass (potentially before module rewriting, even?) that added `@const` annotations to such variables' jsdoc. This would benefit NTI in that namespaces that aren't explicitly annotated as `@const` would get tighter inference. It could also simplify other passes, such as `InlineProperties`, which already builds in a similar check for determining when a property is inlineable.