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/http/header_lookup.wren

@@ -0,0 +1,11 @@
+// retoor <retoor@molodetz.nl>
+
+import "http" for HttpResponse
+
+var headers = {}
+for (i in 0...100) {
+  headers["Header-%(i)"] = "value%(i)"
+}
+var resp = HttpResponse.new_(200, "OK", headers, "")
+System.print(resp.header("header-50")) // expect: value50
+System.print(resp.header("HEADER-99")) // expect: value99