refactor: output preparation, add tests

[ttytm]

Resolves #371

Simplifies output preparation; enables sim testing of the display modules.

As an example I'm sharing an editor screenshot below, to avoid potential issues with displaying utf8 characters in the browser when pasting/linking the code.

[github-actions]

Filename                              Stmts    Miss  Cover    Missing
----------------------------------  -------  ------  -------  --------------------------------------------------------------------------------------------------
src/modules/display/border.rs            47      18  61.70%   20-22, 26-27, 31-33, 37-38, 42-44, 48-50, 105-106
src/modules/display/current.rs           46       6  86.96%   33, 37, 83, 96, 125-126
src/modules/display/day.rs               42       6  85.71%   29, 33, 60, 66, 91-92
src/modules/display/graph.rs            169      59  65.09%   86-125, 141, 148, 155, 160, 176, 182-186, 196, 210, 217, 224, 229, 245, 273-282, 293-315, 332, 368
src/modules/display/gui_config.rs        15       3  80.00%   48, 54, 60
src/modules/display/historical.rs        46      46  0.00%    22-158
src/modules/display/hourly.rs           156      90  42.31%   47-66, 70-79, 88-91, 98, 140, 144-145, 228, 249, 268-278, 321-454
src/modules/display/product.rs           25      25  0.00%    26-74
src/modules/display/utils.rs             38       8  78.95%   14-17, 23, 50, 56-57, 71
src/modules/display/weathercode.rs       31      23  25.81%   13, 17-23, 26-32, 34-41
src/modules/display/week.rs              45       3  93.33%   36, 105-106
src/modules/display/wind.rs              21      11  47.62%   24-27, 35-38, 40-42
src/main.rs                              13      13  0.00%    26-43
src/modules/api.rs                       24      13  45.83%   26-29, 38, 49-88
src/modules/args.rs                       4       4  0.00%    91-95
src/modules/config.rs                    31      25  19.35%   52-97
src/modules/forecast.rs                  20       6  70.00%   7-8, 24-25, 27-28
src/modules/localization.rs             105      41  60.95%   162-203, 230-265
src/modules/location.rs                  53      28  47.17%   25-29, 59-63, 75, 89, 95-104, 109, 118-126, 131-132, 142-143
src/modules/params.rs                    52      52  0.00%    23-133
src/modules/units.rs                     13       0  100.00%
src/modules/weather.rs                   28      28  0.00%    82-153
TOTAL                                  1024     508  50.39%

[github-actions]

Test Results

13 tests  +4   13 ✅ +4   1s ⏱️ ±0s
 1 suites ±0    0 💤 ±0 
 1 files   ±0    0 ❌ ±0 

Results for commit 3d15a4f. ± Comparison against base commit ae0ada5.

♻️ This comment has been updated with latest results.