Add option adjust_baseline to adjust the baseline position on macOS

[ytang]

Add option adjust_baseline to adjust the baseline position on macOS

Fixes #3728

[kovidgoyal]

Some issues:

1. It needs to have a way to specify different units, just like adjust_line_height
2. It needs to work on linux as well look in freetype.c
3. You cant adjust only the baseline you have to adjust the underline position and strikethrough position as well

[ytang]

Some issues:

1. It needs to have a way to specify different units, just like adjust_line_height

Done. Now it takes either a number of pixels or a percentage of the line height.

2. It needs to work on linux as well look in freetype.c

Done.

3. You cant adjust only the baseline you have to adjust the underline position and strikethrough position as well

The underline and strikethrough positions are adjusted accordingly.

PTAL.