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/web/scheduler_integration.wren

@@ -0,0 +1,41 @@
+// retoor <retoor@molodetz.nl>
+
+import "scheduler" for Scheduler
+import "timer" for Timer
+import "web" for SessionStore
+
+var store = SessionStore.new()
+var results = []
+var taskCount = 5
+
+for (i in 0...taskCount) {
+  var index = i
+  Scheduler.add {
+    var session = store.create()
+    session["task"] = index
+    store.save(session)
+    results.add({"id": session.id, "index": index})
+  }
+}
+
+System.print(results.count) // expect: 0
+
+Timer.sleep(50)
+
+System.print(results.count) // expect: 5
+
+var uniqueIds = {}
+for (r in results) {
+  uniqueIds[r["id"]] = true
+}
+System.print(uniqueIds.count) // expect: 5
+
+var hasAll = true
+for (i in 0...5) {
+  var found = false
+  for (r in results) {
+    if (r["index"] == i) found = true
+  }
+  if (!found) hasAll = false
+}
+System.print(hasAll) // expect: true