feat(smd): add grammar rules for basic Markdown elements e.g. headings, emphasis

[nokome]

We currently have a syntax highlighting grammar for Stencila Markdown that covers our extensions: https://github.com/stencila/vscode-extension/blob/main/syntaxes/smd/tmGrammar.yaml.

However it does not yet include rules for many basic Markdown elements. I suggest that we use those defined in VSCode's own Markdown grammar at: https://github.com/microsoft/vscode-markdown-tm-grammar/blob/main/markdown.tmLanguage.base.yaml

Rules that are currently missing from smd/tmGrammar.yaml could be added to the end and then included in the inline and block patterns.