Add aria-label to controls

From https://wordpress.org/support/topic/request-add-aria-label-to-controls/

> I’m using automated tests for my WordPress installation. The controls in the syntax highlighter plugin currently don’t have an aria-label in them. That makes them harder to test, because the numbers in the controls (f.e. inspecter-select-control-2 for the select item to choose plain text, BASH, etc) vary based on the other formatting in the same post. Is it possible to add an aria-label to each input control?

Start by locating the syntax highlighter plugin's input controls and reviewing the linked WordPress support request. Add an aria-label to each control so it can be selected independently of post formatting, then verify that every control exposes a stable accessible label.

Written by the indexing model from the issue text.