Portabletext-html

Fix a bug where a custom block could not be placed directly after a list.

Improve error handling in `renderer.py`.

- Support custom marks with render_text method
- If a block content has multiple custom marks defined, only the first one will be applied

Add a new `render_text` method to `MarkerDefinition`.
The motivation behind adding it is to give a library user more flexibility when they want to render a portable text block, for example:
- a text block is supposed to be rendered under a certain condition
- a content inside a text block is a placeholder

Support user-defined custom marks

Fixed a bug, where user defined custom marks weren't registered by the renderer correctly.

Package re-release under new name.

The package was renamed from `sanity-html` to `portabletext-html`, so the naming is more in line with other portable text-related tools and packages.