Add command substitution

Add command substitution

If a command contains parentheses,
what is inside is executed and its output substitutes it.
Command substitution can work multiples times in a row and nested.
Splicing is automatic:
if the output of the substituted command contains the null character,
it is automatically separated into multiple words.
This is always the desired behavior,
because it cannot appear inside a word.
Inside command substitution,
parentheses and newline lose their special meaning.