Support simpler array update syntax

[swernli]

A major pain point for folks learning Q# (or AI generating it) is the copy-update syntax used for modifying arrays:

mutable a = [1, 2, 3];
let idx = 0;
let update = 0;
a w/= idx <- update; // Sets the first entry to be `0`

We should consider a simpler syntax that is more intuitive while either conforming to current memory copying patterns or introducing new patterns.

One possible solution: we introduce new syntactic suger that is lowered into the same hir::ExprKind::AssignIndex structure such that runtime behavior and analysis via Runtime Capabilities Analysis remains the same. We could support familiar syntax like:

mutable a = [1, 2, 3];
let idx = 0;
let update = 0;
a[idx] = update; // Sets the first entry to be `0`

However, we'd have to be careful about how to handle different forms of indexing (like range-based updates) and arrays of arrays. So something like:

mutable a = [ [1, 2, 3], [1, 2, 3] ];
a[0][0] = 0;

would have to be treated as the equivalent of the older syntax:

mutable a = [ [1, 2, 3], [1, 2, 3] ];
a w/= 0 <- (a[0] w/ 0 <- 0);

Where the older syntax more clearly communicates that a copy of the entire inner array is incurred.