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/concurrent_handlers.wren

@@ -0,0 +1,42 @@
+// retoor <retoor@molodetz.nl>
+
+import "scheduler" for Scheduler
+import "timer" for Timer
+
+var results = []
+var completed = 0
+
+Scheduler.add {
+  results.add("fiber1_start")
+  Timer.sleep(1)
+  results.add("fiber1_end")
+  completed = completed + 1
+}
+
+Scheduler.add {
+  results.add("fiber2_start")
+  Timer.sleep(1)
+  results.add("fiber2_end")
+  completed = completed + 1
+}
+
+Scheduler.add {
+  results.add("fiber3_start")
+  Timer.sleep(1)
+  results.add("fiber3_end")
+  completed = completed + 1
+}
+
+System.print(completed) // expect: 0
+System.print(results.count) // expect: 0
+
+Timer.sleep(10)
+
+System.print(completed) // expect: 3
+System.print(results.count) // expect: 6
+System.print(results.contains("fiber1_start")) // expect: true
+System.print(results.contains("fiber1_end")) // expect: true
+System.print(results.contains("fiber2_start")) // expect: true
+System.print(results.contains("fiber2_end")) // expect: true
+System.print(results.contains("fiber3_start")) // expect: true
+System.print(results.contains("fiber3_end")) // expect: true