Use backticks to represent Markdown

[OAGr]

Description of suggestion or shortcoming:

• Add a new string type, using "`" marks.
• For this new string type, multiple lines should be supported.
• When done, only show this new format as Markdown. The previous format should render as strings.
• We could distinguish this using a Tag or just a different type.

[berekuk]

Marking this as epic, because:

• it's potentially a breaking change that will require some care to upgrade
• needs support on the components side (syntax highlighting, preferably with embedded markdown highlighting)
• figure out new practices for @notebooks
  • do we want to continue doing them as arrays of separate lines and string concatenation?
• can we implement # Result: ${var} interpolation? This would be cool but requires parser modes; Lezer can handle this, but I'm not sure about Peggy
  • this can be done later, but that's technically another breaking change

[berekuk]

Other related issues: #2241, #2760.