add debug alias for let

[patricklx]

This helps especially on cases for e.g.
When one has a let var which collides with a html element

{{#let x as |b|}}
  {{b}}
  <b\>
{{/let}}

[NullVoxPopuli]

Any ideas an the perf check failing? (It's failing on all branches)

We can't really merge anything until that's fixed :(

[NullVoxPopuli]

Seems good

Dismissing because we want to go more in a direction that exposes what the user typed, rather than try to derive debug aliases / names from existing runtime information --- here is an example: #1634 -- @wycats is currently thinking about this PR and is actively workingish on it -- this way when folks import something and alias it, we use the alias name, rather than the name it was defined with (current behavior as implemented)

[patricklx]

@wycats i saw you are working on this. Maybe this us already fixed?

[patricklx]

Btw, this is not derived debug alias, but the actual debug symbol what the user typed