[rtext] Add support for Em Box font size metric (Supersedes #5949)

[thiagomucci]

Fixes #5678. Supersedes #5949 with C99 coding conventions, TRACELOG parameter fix, and open-source font assets.

[raysan5]

@thiagomucci It seems required font assets and its licenses info are not included

[thiagomucci]

Updated text_font_size_metric.c to use standard raylib font assets comparing EM_BOX vs ASCENT metrics side-by-side. Unlicensed font assets from PR #5949 have been removed

[raysan5]

@thiagomucci oh! great! thanks for the review! could you add the .png screenshots por the example? It can be captured with F12 while running the example.

[raysan5]

I'm reviewing this change but I got some concerns, raylib expects to use font size in pixels for the font loading but also for all the text drawing functions. SetFontSizeMetric() could confuse library users to switch to EM_BOX to all font size definitions, including text drawing, creating undesired results.

One solution could be checking it on all text drawing functions but I think it will be even more confusing... Not sure about this change...