Improve variable brace expansion
This will fix the issue of variables not being expanded if there's more than one variable expansion within a word. It will also allow using colons to separate variable expansions, without requiring braces. Tests have also added to ensure that the functionality works.
$ let FOO = foo
$ let BAR = bar
$ echo $FOO:$BAR
foo:bar
$ echo ${FOO}:${BAR}
foo:bar