Timer mode

[dylwhich]

Description

Simple timer mode, has stopwatch and countdown modes. Keeps previous time saved in NVS at exit.

Test Instructions

A starts/resumes, B stops/resets, up/down change the set time, and left/right change between stopwatch and timer.

Ticket Links

Readiness Checklist

• I have run make format to format the changes
• I have compiled the firmware and the changes have no warnings
• I have compiled the emulator and the changes have no warnings
• I have run make cppcheck and checked that cppcheck_result.txt has no warnings for the changes
• I have added doxygen comments to any code used by more than one Swadge mode. This includes /*! \file comments with Design Philosophy, Usage, and Example sections for new headers.
• I have run make docs and checked that doxy_warnings.txt has no warnings for the new code

[AEFeinstein]

Just from testing, not from looking at the code, two comments

1. The text is off-center. Please align that for my OCD
2. The test instructions on the PR are great, but having them on the display would be even greater. I can figure out the controls by playing with it, but instruction labels (like in Jukebox or Tunernome) are really nice and welcoming

[dylwhich]

Okay, I think everything has been addressed now. Here's the new look:

[AEFeinstein]

Oh this is infinitely more useable. I'm gonna merge this now, but I'll casually mention two stretch goals if you're feeling stretchy

1. Some sort of buzzer and/or LED indication when the timer runs out
2. Finer-grain increments for the timer. You could do that thing where the longer you hold the button, the faster it increments, or it switches from 1s increments to 30s increments. I think that's how my stove timer works.