AK: Remove default parameter from StringBuilder's constructor

This separates the StringBuilder constructor into 2 constructors. This
essentially allows forming code of the following pattern:

    StringBuilder foo() { return {}; }

Otherwise, we would get the following compiler error:

    chosen constructor is explicit in copy-initialization

Due to the explicitness of the StringBuilder constructor.

This is required for an upcoming update to ICU 76, where we use our
StringBuilder in templated ICU code.

AK/StringBuilder.cpp

@@ -40,6 +40,11 @@ ErrorOr<StringBuilder> StringBuilder::create(size_t initial_capacity)
     return StringBuilder { move(buffer) };
 }
 
+StringBuilder::StringBuilder()
+    : StringBuilder(inline_capacity)
+{
+}
+
 StringBuilder::StringBuilder(size_t initial_capacity)
     : m_buffer(MUST(create_buffer(initial_capacity)))
 {

AK/StringBuilder.h

@@ -23,7 +23,8 @@ public:
 
     static ErrorOr<StringBuilder> create(size_t initial_capacity = inline_capacity);
 
-    explicit StringBuilder(size_t initial_capacity = inline_capacity);
+    StringBuilder();
+    explicit StringBuilder(size_t initial_capacity);
     ~StringBuilder() = default;
 
     ErrorOr<void> try_append(StringView);