New key-value function: get(kv, k, default)

[nahiyan-yasaar-selise]

Currently, the behaviour of kv[k] is to raise an error when a key is not found. To avoid this, we are forced to create a separate array which has a copy of all the keys available in the kv, after which we can use the ternary operator in an expression node: x = has(arr, k1) ? kv[k1] : 0. For a single item it's not a big deal, but we have many such items (e.g. y checks for k2 and z checks for k3 etc).

We would like the ability for it to return null instead. Since error handling is important, the current default behaviour for this syntax (kv[k]) can stay as is. Instead, it would be better to have a separate function so we can do x = get(kv, k1) ? get(kv, k1) : 0.
Even better is if we have an optional parameter for returning default values when a key isn't found, so we can simplify it even more to x = get(kv, k1, 0)

For reference, this is identical to python's dict[key] (raises error for missing key) syntax vs dict.get(key, default) syntax.