CSSOM: Tackle nested @import stylesheets

[jeeyyy]

As an enhancement to the work done on PR - #958, which aims to fetch and resolve all CSSStyleSheets that are inline, relative and are cross-domain, there is a need to parse CSSStyleRules from each sheet to understand the usage of @import statements to refer to relative or cross-domain sheets, to ascertain if they are to be fetched.

These references can happen at various nested levels and a returned result/ stylesheet of an @import resource could lead to further @imports and so on, so care should be taken to handle infinite recursion.

The issues to tackle are:

• nested @import rules at multiple levels (eg: nested @import inside an @import(ed) resource).

• media/ conditionals to @import or link rules -https://developer.mozilla.org/en-US/docs/Web/CSS/@import

• @import inside of media queries

[jeeyyy]

It is worth tackling this, only after #1113 is merged.