Incorporate global/page level configuration into the spec

[chrisjsewell]

Currently, there are numerous configuration formats/schemas for how MyST is parsed/rendered across implementations:

global:

• myst-parser (+ sphinx) conf.py: https://myst-parser.readthedocs.io/en/latest/configuration.html#global-configuration
• myst-nb conf.py: https://myst-nb.readthedocs.io/en/latest/configuration.html#global-configuration
• jupyter-book _conf.yml: https://jupyterbook.org/en/stable/customize/config.html
• mystjs myst.yml: https://js.myst.tools/guide/frontmatter#in-a-myst-yml-file
• vscode myst plugin setting.json (to set markdown-it syntax plugins): https://github.com/executablebooks/myst-vs-code/blob/c5b209c77196a46152f545ff366d8b729f7b9c84/package.json#L125
• jupyterlab-myst: setting of markdown-it syntax plugins via jupyterlab-markup config

page level:

• myst-parser: https://myst-parser.readthedocs.io/en/latest/configuration.html#local-configuration
• myst-nb: https://myst-nb.readthedocs.io/en/latest/configuration.html#local-configuration
• mystjs: https://js.myst.tools/guide/frontmatter

For consistency across tools, it would be ideal to have a defined "core" JSON schema for such configuration, then potentially also a standard file name/format for global configuration (although this may not be possible for all implementations).

Particularly for page-level metadata, namespacing should definitely be considered, and also potentially defined way for extensions to add configuration.