Add font style rules for markdown

[ziw-liu]

Adding font styles to several textmate scopes in markdown:

• Heading (bold)
• Inline bold (Bold)
• Inline italic (italic)

[mvllow]

These changes should go in _pinecone-color-theme.json and then we can build all the variants with npm run build :)

[mvllow]

Thank you! We'll build and release asap