plusparse/inline_parser library

Inline parser: turns a run of text into inline MdNodes (bold, italic, code, links, images, LaTeX, source tags, …). Single forward pass over the characters; emphasis content is parsed recursively. Ported 1:1 from the Rust plusparse inline parser, using code-unit scanning and indexOf instead of a char vector (all delimiters are ASCII, so this is safe and fast on Dart's UTF-16 strings).