[Enhancement][Postcss] : Requesting to add postcss-value-parser and postcss-selector-parser for postcss transformer

**Current**
Currently, the postcss parser doesn't provide this out of the box. So most of the `value` node of the graph remains as it is like the value written.

**Enhancement**
There is a [`postcss-value-parser`](https://github.com/shellscape/postcss-values-parser/tree/master) already present I was thinking if it can be added as an opt-in thing so that those values can be transformed to nodes.

**Alternative**
I couldnt find any other parser or default from postcss.

But I do think it will help a lot to those who are writing postcss-plugins

Start by locating the PostCSS transformer integration and reviewing how parser output is represented. Check the postcss-value-parser and postcss-selector-parser references in the issue, then define the opt-in behavior and verify that values and selectors are exposed as transformed nodes without changing the default output.

Written by the indexing model from the issue text.