feature: collapse sections

[ha1ix]

Did you check the tree-sitter docs?

• I have read all the tree-sitter docs if it relates to using the parser

Is your feature request related to a problem? Please describe.

Long documents become difficult to grok all at once, often requiring external tables of contents to organize

Describe the solution you'd like

Command to collapse/expand content under a given header/sub-header and/or collapse/expand all

Describe alternatives you've considered

There is a markdown LSP called IWE which has this feature but has other unnecessary features

Additional context

Thank you!