proposal: feature: allows user to specify font theme without having to override all components

[nrcaz]

Context:

My app is using a dark theme, the text by default is black, having the possibility to set a default font theme instead of rewriting default components would be nice

Solution:

Added a new props called theme which can take most text props for all text block types

 <PortableText value={value} theme={{normal: {color: 'white', fontFamily: 'Arial'}, h1: {color: 'white', fontFamily: 'Arial', fontSize: 32, fontWeight: "700"}}} />

I had to be able to specify fontWeight since on Android not controlling the weight can mess up with the fontFamily.

Since i didnt wanted to create breaking change with the use of defaultComponents i had to create a new function getDefaultComponentsWithTheme

PS: also fixed a typescript issue for locating types file due to index.d.ts file not being specified in package.json