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/error_isolation.wren

@@ -0,0 +1,46 @@
+// retoor <retoor@molodetz.nl>
+
+import "scheduler" for Scheduler
+import "timer" for Timer
+
+var results = []
+var errors = []
+
+var handleSafe = Fn.new { |task|
+  var fiber = Fiber.new {
+    task.call()
+  }
+  var error = fiber.try()
+  if (error != null) {
+    errors.add(error)
+  }
+}
+
+Scheduler.add {
+  handleSafe.call {
+    results.add("good1")
+  }
+}
+
+Scheduler.add {
+  handleSafe.call {
+    Fiber.abort("intentional error")
+  }
+}
+
+Scheduler.add {
+  handleSafe.call {
+    results.add("good2")
+  }
+}
+
+System.print(results.count) // expect: 0
+System.print(errors.count) // expect: 0
+
+Timer.sleep(10)
+
+System.print(results.count) // expect: 2
+System.print(errors.count) // expect: 1
+System.print(results[0]) // expect: good1
+System.print(results[1]) // expect: good2
+System.print(errors[0]) // expect: intentional error