chore: modernize CI and update Go toolchain

- Bump Go from 1.19 to 1.26 and update all dependencies
- Rewrite CI workflow with matrix strategy (Linux, macOS, Windows)
- Update GitHub Actions to current versions (checkout@v4, setup-go@v5)
- Update CodeQL actions from v1 to v3
- Fix cross-platform bug in mock/path.go (path.Join -> filepath.Join)
- Clean up dependabot config (weekly schedule, remove stale ignore)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

vendor/github.com/alecthomas/chroma/v2/serialise.go

@@ -150,8 +150,12 @@ func NewXMLLexer(from fs.FS, path string) (*RegexLexer, error) {
 
 		regexAnalysers := make([]regexAnalyse, 0, len(config.Analyse.Regexes))
 
+		regexFlags := regexp2.None
+		if config.CaseInsensitive {
+			regexFlags = regexp2.IgnoreCase
+		}
 		for _, ra := range config.Analyse.Regexes {
-			re, err := regexp2.Compile(ra.Pattern, regexp2.None)
+			re, err := regexp2.Compile(ra.Pattern, regexFlags)
 			if err != nil {
 				return nil, fmt.Errorf("%s: %q is not a valid analyser regex: %w", config.Name, ra.Pattern, err)
 			}