refactor `noteworthy` into subpackages

[benrbray]

At the moment, noteworthy is organized in a very electron-specific way, but I would like to additionally support the following:

• #17
• embed the Noteworthy frontend as a VS Code extension for wysiwyg editing of local markdown files
• frontend-only version of the editor that can be hosted on e.g. my website as a demo of the editing experience

To do this, I will refactor noteworthy into several parts, organized as a monorepo using pnpm workspaces:

• @noteworthy/markdown for remark extensions / config /
• @noteworthy/editor for anything related to ProseMirror, including the Remark AST <-> ProseMirror Doc conversion, as well as wrapper types around core ProseMirror functionality to address #31
• @noteworthy/electron for the electron-specific bits
• @noteworthy/extensions for extensions to the editor

This will also make it easier to test individual components of Noteworthy in isolation.

Related Issues

• #31
• #34