Basic parameter expansion semantics

[magicant]

• Add get_variable to expansion Env
• Expand unset variable with ParamRef
• Split Expander into Env and Output = (Expansion, is_quoted)
• Expand existing variable with ParamRef
• Use ParamRef from TextUnit::expand

[magicant]

Split Expander into Env and Output = (Expansion, is_quoted)

This is needed to pass a borrow from Env to Expansion. As long as we borrow the variable value from Env via Expander, we cannot mutably borrow the Expansion via the same Expander.