Markdown Notebook Panes

Nobody has claimed this yet.

Pre-requisite: #997

Spec in progress in https://github.com/microsoft/terminal/blob/dev/migrie/s/north-star/doc/specs/NorthStar/Markdown%20Notebooks.md

> ### User Stories
>
> * **A**: The user can perform some commandline action (like `wt open
> README.md`), which opens a new pane in the Terminal, with the markdown file
> rendered into that pane.
> * **B**: Markdown panes have buttons next to code blocks that allow the text of
> that block to be sent directly to the adjacent terminal as input.
> * **C**: The user can press different buttons to run some subset of all the
> commands in the file
> - **C.1**: Run all the commands in this file
> - **C.2**: Run all the commands from (the currently selected block) to the end
> of the file
> - **C.1**: Run all the commands from (the currently selected block) to the
> next header. (e.g., run all the commands in this section of the doc.)
> * **D**: The user can edit the contents of the markdown file directly in the
> Terminal.
> * **E**: The Terminal could be configured to automatically open a markdown file
> when `cd`ing to a directory
> * **F**: The command for opening a markdown file also supports opening files
> from the web (e.g. directly from GitHub)
> * **G**: Code blocks in the markdown file are automatically assigned
> autoincrementing IDs. The user can perform an action (via keybinding, command
> palette, whatever) to execute a command block from the file, based on it's
> assigned ID.
> * **H**: ...

_maintainer notes: original OP below the break_

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# Description of the new feature/enhancement

For many developers, especially embedded developers, they would pre-define many text commands and save them in a text file. If WT can split views and open and display text in one view, we can quickly send the text commands to the terminal. I can send lines, send selected blocks, or send the entire content of the file, and preset some shortcuts.

# Proposed technical implementation details (optional)


Start by reviewing prerequisite issue #997 and the in-progress North Star Markdown Notebooks specification linked in the issue. No implementation files or tests are named; the work needs a narrowed, agreed user story and acceptance criteria before a newcomer can identify what done means.

Written by the indexing model from the issue text.