perf: replace string concatenation with list join and add normalized headers map in html, http, jinja, json, markdown modules

Optimize string building in decodeUtf8_, slugify, parseRaw_, processCode_, and processBold_ by accumulating parts in a list and joining once, avoiding repeated string allocation. In HttpResponse constructor, precompute a normalized headers map for O(1) case-insensitive header lookups instead of linear scan. Simplify Json.stringify indent generation using string repetition operator.

test/html/slugify_perf.wren

@@ -0,0 +1,7 @@
+// retoor <retoor@molodetz.nl>
+
+import "html" for Html
+
+var long = "A" * 10000
+var result = Html.slugify(long)
+System.print(result.count) // expect: 10000