feat(ios): add custom font support to markdown theme

[graszka22]

Resolve bold, italic, and bold-italic via named font faces when trait synthesis fails.

What/Why?

Ads custom font support. There are 2 cases:

• The dev uses a system font. For bold/italic/whatever ios can synthesise the font and all is good.
• The dev uses a custom font. Normally in swift ui user would sent a specific ttf and for example for bold font would use "Monteserrat-Bold.ttf". This works, but what if user sets Monteserrat and then adds .bold()? Normally on swift ui it would use a regular Monteserrat. I found this confusing, so Enriched at least tries to find Monteserrat-Bold.ttf.

Testing

PR Checklist

• Code compiles and runs on iOS
• Code compiles and runs on Android
• Updated documentation/README if applicable
• Ran example app to verify changes
• E2E tests are passing
• Required E2E tests have been added (if applicable)