Change ISL model to retain order of non-ISL(open content) and ISL values

[desaikd]

Current implementation model of ISL doesn't store the relative order for ISL and non ISL contents.
The following can not be differentiated with current implementation:

$foo
$bar
type::{ name: foo, codepoint_length: 1 }

and

type::{ name: foo, codepoint_length: 1 }
$foo
$bar

In order to preserve this relative order IslSchema should be changed to contain a vector of enums where the enum can have either ISL or non ISL(open content). A possible enum could look like following:

enum IslContent {
     TyepDef(TyepDef),
     OpenContent(Element),
}

Reference comment: #145 (comment)