Flush errors after namer and resolver

[ilyailya]

The PR flushes errors after running the namer and the resolver.
The change also adds cache for the errors, preventing Sorbet from losing errors due to additional flushes

Motivation

This would make Sorbet feel more responsive, as errors would be displayed more quickly in LSP mode.

I tested the PR on Stripe's codebase by inserting a specific piece of code and tracking the timing of error reports through LSP logs

    include # this is a namer error

    B = T.type_alias { String }


    class A < B # this is resolver error
    end

Error flushing mode	Time until Namer error, s	Time until Resolver error, s
Only after typecheck	12.29	12.29
After namer, resolver and typechek	6.3	11.4

Test plan

See included automated tests.