Extend 'CSSBuilder' to Support In-Memory Config

[dmtrshat]

Description

Add new methods or constructor in CSSBuilder that accept ConfigInMemory (it's better to create new CSSBuilder constructor or a separate impl). This means:

• Parsing/Collecting Classes (if needed):
• Replace file reads with direct iteration over the provided strings in ConfigInMemory.projects.
• Possibly reuse the same parsing logic (Parser) by adding a method like parse_classes_from_string(content: &str).
• Minification/Optimization:
  • Reuse CSSOptimizer but feed it the concatenated or generated string from memory, not from files.
  • Return or store compiled CSS in some internal structure (e.g., (output_name, compiled_css)).

Acceptance Criteria

1. A new function, for example, CSSBuilder::new_in_memory(config: &ConfigInMemory, optimizer: &dyn CSSOptimizer, ...) -> Self (or an equivalent approach) is implemented.
2. The method reads CSS from content: Vec<String> rather than file paths.
3. For each in-memory project, returns (or stores) a compiled CSS string associated with output_name.
4. Existing file-based constructor or build() remains functional and unbroken.