lang: Unified block string syntax

[nhooyr]

Present

x: |md
  # markdown
|

Proposed

x: md"
  # markdown
"

x: md'
  # markdown
'

Means no new symbol for block strings and you can choose between " and ' for your quoting needs just like regular strings.

We ought to also allow making the terminating symbol for a string more than a quote just like Rust and Swift allow.

[alixander]

maybe we should make a milestone like "next major version" to include issues like these

[nhooyr]

Go for it!