ProseMirror Basic Schema

Provides the default rich-text schema for ProseMirror, including nodes (doc, paragraph, blockquote, heading, code_block, image, hard_break, horizontal_rule, text) and marks (link, em, strong, code). Current version 1.2.4 is stable and mature with infrequent updates. Key differentiator: designed to be extended or subsetted in custom schemas via `schema.spec.nodes` and `schema.spec.marks`. ES module only; no CommonJS or UMD bundle. Ships TypeScript definitions.