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 cding 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

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)