Add the ability to remove and list aliases

[Soumil-07]

alias without any arguments now lists all set aliases. unalias <alias> can unset aliases.

[sophiajt]

I like the alias listing the aliases. There's also $scope.aliases. Should the first be another way to call it? Might make sense to consolidate at least.

Before we land this, I also wanted to think through how we want this to work when there are blocks that use the alias which is later unaliased. Say something like:

alias ll = ls -l
let xyz = { ll -a }
unalias ll
do $xyz

The alias was valid when the block was created, but later wouldn't be when we call xyz.

[fdncred]

From what I saw on discord, in the example above, xyz still functions even after the unalias. I'd like to move forward with this. What do you think @jntrnr ?

[sophiajt]

Great, yeah if that still works I think that's something we could keep working that way.

@Soumil-07 - can you add a few test cases so we can make sure we keep this working? It'd also be good to have a test case for using unalias and then getting an error after that because the alias is no longer available