Refactor: Extract formatting functions and improve type safety (#197)

This is a re-creation of #191 

## Summary
This PR refactors the CSS formatting library to extract lower-level
formatting functions as public exports and improves type safety by using
type guards instead of node type constants.

## Key Changes

- **Extracted public formatting functions**: `format_value()`,
`format_declaration()`, `format_selector()`, `format_atrule_prelude()`,
and `unquote()` are now exported as standalone functions that can be
used independently of the main `format()` function
- **Replaced type checking with type guards**: Changed from checking
`node.type === NODE.CONSTANT` to using type guard functions like
`is_function()`, `is_declaration()`, `is_rule()`, etc., improving type
safety and reducing reliance on magic constants
- **Updated imports from css-parser**: Now imports type guards and type
definitions directly from `@projectwallace/css-parser` instead of node
type constants
- **Improved function signatures**: Formatting functions now accept
`FormatOptions` parameter for consistent minification control across all
formatters
- **Updated dependencies**: Bumped `@projectwallace/css-parser` from
`^0.13.5` to `~0.14.7` to support new type guards and types
- **Enhanced test coverage**: Added comprehensive tests for all newly
exported functions including `format_atrule_prelude()`,
`format_selector()`, `format_declaration()`, `format_value()`, and
`unquote()`
- **Updated documentation**: Added README section documenting the new
partial formatters API

## Implementation Details

- The refactored functions maintain backward compatibility with the
existing `format()` and `minify()` APIs
- Type guards provide better IDE support and compile-time safety
compared to runtime type checking
- All formatting logic remains unchanged; this is purely a refactoring
for better API design and maintainability
- Version bumped to 3.0.2 as a minor feature release

https://claude.ai/code/session_01M2d2VuMLMKQSFTL7GniQny

Co-authored-by: Claude <noreply@anthropic.com>

Author: bartveneman

README.md

@@ -52,6 +52,38 @@ Need more examples?
 - [StackBlitz example using CommonJS](https://stackblitz.com/edit/stackblitz-starters-phchci?file=index.js)
 - [StackBlitz example using ES Modules](https://stackblitz.com/edit/stackblitz-starters-hrhsed?file=index.js)
 
+### Partial formatters
+
+The package also exports lower-level formatters for individual CSS constructs:
+
+```js
+import {
+  format_value,
+  format_declaration,
+  format_selector,
+  format_atrule_prelude,
+} from '@projectwallace/format-css'
+
+// Format a CSS value (e.g. the right-hand side of a declaration)
+format_value(node.value)
+
+// Format a single CSS declaration (property + value)
+format_declaration(node)
+
+// Format a single CSS selector
+format_selector(node)
+
+// Format an at-rule prelude string (e.g. the query part of @media)
+format_atrule_prelude(node.prelude.text)
+```
+
+All of these accept the same options as `format()`. However, `tab_size` has no effect on them since they do not produce indented output.
+
+```js
+format_declaration(node, { minify: true })
+format_selector(node, { minify: true })
+```
+
 ## Formatting rules
 
 1. Every **AtRule** starts on a new line