Add repl preview

Repl now has an additional mode of evaluation where observable (visible
outside of the evaluated expression) side effects are not allowed. More
specifically all assignments and reassignments are *prohibited* unless
they fall in the following categories:

- Assignments and reassignments within the top level environment
- Assignments and reassignments for the environments created during
  evaluation of the expression being previewed

Examples:

    let repl = BQN.makerepl()
    repl.preview('42') // ok, pure
    repl.preview('x ← 42 ⋄ x + 1') // ok, x is from the top env

    repl('f ← {x ← 0 ⋄ F ← {x+↩𝕩} ⋄ f}')
    repl.preview('F 42') // fail! mutates x

How it's implemented:

- A global `preview` flag determines if we are in "preview" mode
- During compilation of bytecode to JS we inject code which marks
  environments with `inpreview` flag - `true` means they've been created
  during "preview".
- Top level environment always has `inpreview = true` as we have this
  handled by restoring it to the values before the "preview" eval
  started.
- While setting variables we check if are in "preview" and if the target
  environment has `inpreview` flag, otherwise we fail.

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