Highlight unused portions of code on compile

I among other programmers would find it helpful to highlight unused portions of code on compile/verify with respect to particular values not changing under certain conditions e.g. not a value retrieved from EEPROM or an external source such as input pins or a serial command.

Start by locating the compile/verify path in the Arduino IDE and determine where compiler diagnostics could be converted into highlighted code. The issue does not name files or tests, so the intended analysis and the conditions for considering a value unused need to be defined before implementation. Done should include highlighting unused portions while respecting values that may come from EEPROM, input pins, or serial commands.

Written by the indexing model from the issue text.