Letter spacing support

We greatly improved the way we handle text layers: you can now adust the letter spacing. The letter spacing unit is percents of the font size.

Other improvements & fixes

• Converted line height unit to percents
• Added examples to the gallery
• Improved performance to display examples from the gallery
• UI improvements on the Figma plugin
• Text can now be imported natively from Figma
• Text with missing fonts can now be converted to native text by replacing or uploading the font