variable substitution $var and ${var}

[christianparpart]

support variable substitution

when declaring a variable like

set foo bar

then it should be able to be used when parsing a statement again, like:

some-command $foo

but also within strings itself

echo "Hello $foo."

It may make sense to support variable substitution in the bash-like syntax as well: ${variable_name}.

We should not yet think about supporting extended just yet (e.g. ${foo:-bar} etc)