feat: var declarations

[Chriscbr]

This change introduces the var modifier, which indicates that a variable can be reassigned to. For example:

let var sum = 0;
for item in [1, 2, 3] {
  sum = sum + item;
}

Partially addresses #109 (only implements the modifier for variables, not for class/resource fields)

Also fixes a bug where expressions used in an interpolated string are not recognized as captures.

By submitting this pull request, I confirm that my contribution is made under the terms of the
Monada Contribution License.

[eladb]

The compiler doesn't yet check that let variables cannot be reassigned to, but this has not been an issue to date in preflight since it is translated into const in JavaScript code.

Do we have a tracking issue for this?

[Chriscbr]

Not yet. Will create

Also I need to add a test that "var" symbols cannot be captured (right?)

[Chriscbr]

Awaiting feedback from @yoav-steinberg

[monadabot]

Congrats! 🚀 This was released in Wing 0.4.106.