Linter: Introduce ControlFlowTrackingVisitor to handle control flow

[marcoroth]

This pull request introduces a new ControlFlowTrackingVisitor base class which allows linter rules to track state across branches and loops.

With this new visitor we are now able to track state more strategically in branches.

interface MyRuleState {
  counters: Map<string, number>
  flags: boolean[]
}

interface MyBranchState {
  savedCounters: Map<string, number>
}

class MyCustomRuleVisitor extends ControlFlowTrackingVisitor<MyRuleState, MyBranchState> {
  protected onEnterControlFlow(controlFlowType: ControlFlowType, wasAlreadyInControlFlow: boolean): MyRuleState {
    return { counters: new Map(), flags: [] }
  }

  protected onExitControlFlow(controlFlowType: ControlFlowType, wasAlreadyInControlFlow: boolean, stateToRestore: MyRuleState): void {
    this.processCounters(stateToRestore.counters)
  }
}

Using this new infrastructure, we are now able to refactor the html-no-duplicate-ids rule to not flag IDs when they are in mutually exclusive branches.

This now doesn't get flagged anymore:

<% if some_condition? %>
  <span id="id-1">content1</span>
<% else %>
  <span id="id-1">content2</span>
<% end %>

This resolves #441 and resolves #273.