[Feature] Declaring empty variables

**Is your feature request related to a problem? Please describe.**

In bash, it is possible to declare an empty variable, like so:

#!/usr/bin/env bash

declare foo
foo="bar"
echo "$foo"

Outputs:

bar

However, this functionality is not present in Amber:

main {
let foo
foo = "bar"
echo foo
}

Outputs:

ERROR Expected '=' after variable name 'foo'
at empty.ab:3:6

2| let foo
3| foo = "bar"
4| echo foo

**Describe the solution you’d like**

Simply, compile `let foo` to `declare foo`.

Additionally, `declare` is useful for enforcing variable type, as demonstrated in [this](https://unix.stackexchange.com/a/510342) stackexchange answer. This could be written in Amber as `let foo = Text`.

**Describe alternatives you’ve considered**

I *suppose* writing `let foo = ""` would suffice, but my proposed solution implements more bash functionality and allows for cleaner Amber code.