[BUGFIX] Fix SCSS compilation, cache handling and import resolution

[davidk1982]

Summary

Fixes SCSS compilation that was broken due to incompatibilities with scssphp v2 (Dart Sass).
Removes the complex Importer system in favor of a simplified, working approach using
compileString() with variable injection as SCSS source code.

Changes

Compiler.php — Complete rewrite

• Variable injection via SCSS declarations instead of addVariables() with Value objects.
  scssphp v2 requires Value objects for addVariables(), which cannot handle SCSS expressions
  like $line-height-base - .25. Injecting as SCSS source code evaluates them in context.
• Two-phase variable injection: Literals/internal expressions prepended before import
  (override !default), external $var references appended after import (defensive fallback).
• Path value quoting: Values starting with / are auto-quoted to prevent parse errors
  (e.g. /_assets/... paths from EXT: resolution would be interpreted as division).
• compileFile() → compileString(): Required for source-level variable injection.
• Import paths: dirname-based, EXT: callable resolver, and vendor directory path.
• Cache key fix: Includes cssFilePath so different output targets get separate cache entries.
• File existence check: Verifies CSS file exists before returning cached path.

RenderPreProcessorHook.php

• Added declare(strict_types=1) and typed properties
• v13-compatible TypoScript access via $GLOBALS['TYPO3_REQUEST']->getAttribute('frontend.typoscript')
• Added EXT: variable resolution (resolves EXT:ext/... → /_assets/{hash}/... web paths)
• Fixed parseBooleanSetting() null handling
• Added outputfile path handling (compress=false + path normalization)

Removed (unused/broken with scssphp v2)

• Classes/ImportResolver.php
• Classes/Importer/ExtensionFilesystemImporter.php
• Classes/Importer/FilesystemImporter.php
• Classes/Importer/VariableFilesystemImporter.php

Technical Context

scssphp v2 (Dart Sass compliant) introduced breaking changes:

• addVariables() requires Value objects — raw strings no longer accepted
• Compiler constructor takes no parameters (no cache options)
• Strict unit math: unitless - em operations are invalid
• Variable evaluation is eager (at declaration time, not lazy)

The Importer-based approach from the original v13 code did not correctly handle
TypoScript variable expressions and produced incorrect CSS output (literal $variable
strings in CSS).

Note on v14 compatibility

The release/v14 branch (14.0.4) has the same underlying bugs in its variable handling:

• ValueConverter::fromPhp() produces quoted strings for SCSS expressions
• SCSS expressions like $line-height-base - .25 are not evaluated, output as literal strings
• Cache key does not include output file path

The approach used in this PR (SCSS source injection + two-phase ordering) is directly
applicable to v14 as well. A separate PR for release/v14 could port this fix with
minimal adaptation (mainly the ScssViewHelper refactoring and composer constraints).

Testing

Tested with Bootstrap 4 SCSS compilation via theme extensions using:

• Literal color values, rem/px units
• SCSS expressions referencing other TypoScript variables ($font-size-base * 1.875)
• EXT: path resolution for font/icon variables
• Multiple output targets (outputfile TypoScript option)
• Source maps (inline)