Semibold 'above' and 'aboveMark' mark positioning

[kopeckyf]

This is a continuation of #426. The goal of this PR is to make mark-to-mark anchors available for some combining diacritics, and also to add anchors for some Latin glyphs that don't have reserved accented glyph slots in the Unicode table.

Goals:

• [x] Add above marks in Serif-Semibold: U-025B ɛ and U-254 ɔ
• [x] Add above marks in Serif-Semibold: U-1E7C Ṽ, U-1E7D ṽ, and U-00F5 õ
• [x] Add above marks in Serif-Semibold: U-1EBD ẽ, U-0169 ũ, U-0129 ĩ, U-00E3 ã
• [x] Add above marks in Serif-Semibold and Serif-Italic: U-0292 ʒ
• [x] Add above marks in Serif-SemiboldItalic: U-025B ɛ and U-254 ɔ
• [x] Add above marks in Serif-SemiboldItalic: U-1E7C Ṽ, U-1E7D ṽ, and U-00F5 õ
• [x] Add above marks in Serif-SemiboldItalic: U-1EBD ẽ, U-0169 ũ, U-0129 ĩ, U-00E3 ã
• [x] Add aboveMark base marks and marks in Serif-Semibold:
  • U-0302 circumflex
  • U-030C caron
  • U-0304 macron
  • U-0301 acute
  • U-0300 grave
  • U-0303 tilde
• [x] Add aboveMark base marks and marks in Serif-SemiboldItalic:
  • U-0302 circumflex
  • U-030C caron
  • U-0304 macron
  • U-0301 acute
  • U-0300 grave
  • U-0303 tilde

New goals in 2022:

• [x] Add above mark for U-028A ʊ, U-1ECC/U-1ECD Ọ/ọ, U-0186 Ɔ, U-1EE4/1EE5 Ụ/ụ in Serif-Semibold.

[alerque]

The conflicts here will need to be resolved similarly to what I just did for #467, but since the resolution will change after that PR merges I'll hold off until you consider it ready for review or that PR is merged, whichever comes first.

[kopeckyf]

I also corrected the above anchor for Q which was slightly off to the left.

[alerque]

Thanks for the contribution, and sorry it took so long to review. I hope I've handled the merge conflicts on this correctly. The merge conflicts should be resolved in c2d82f3 then this merged back in f188c77.

[kopeckyf]

Thank you very much for merging this, @alerque!

In another thread I read that there might be an alternative approach to mark placement that is more automatic and more reliable than what I have done in this PR.

@alerque, is there any plan in place for future adjustments of mark positioning in Libertinus? If I want to make more contributions, should I pursue the approach that I have taken in this PR, i.e., manual placement of anchors?

Some issues with mark placements remain in Libertinus (but I can't tell how many there are). I have a few more changes locally and I'd be happy to send a new PR for these individual improvements, too. But if there was some automatic way to place marks, I'd be happy to stop and use this alternative approach instead.