Add functions for each Value case (#9736)

# Description
This PR ensures functions exist to extract and create each and every
`Value` case. It also renames `Value::boolean` to `Value::bool` to match
`Value::test_bool`, `Value::as_bool`, and `Value::Bool`. Similarly,
`Value::as_integer` was renamed to `Value::as_int` to be consistent with
`Value::int`, `Value::test_int`, and `Value::Int`. These two renames can
be undone if necessary.

# User-Facing Changes
No user facing changes, but two public functions were renamed which may
affect downstream dependents.

crates/nu-parser/src/eval.rs

@@ -12,7 +12,7 @@ pub fn eval_constant(
     expr: &Expression,
 ) -> Result<Value, ParseError> {
     match &expr.expr {
-        Expr::Bool(b) => Ok(Value::boolean(*b, expr.span)),
+        Expr::Bool(b) => Ok(Value::bool(*b, expr.span)),
         Expr::Int(i) => Ok(Value::int(*i, expr.span)),
         Expr::Float(f) => Ok(Value::float(*f, expr.span)),
         Expr::Binary(b) => Ok(Value::Binary {