AK: Add String::copy_characters_to_buffer()

This is a strcpy()-like method with actually sane semantics:

* It accepts a non-empty buffer along with its size in bytes.
* It copies as much of the string as fits into the buffer.
* It always null-terminates the result.
* It returns, as a non-discardable boolean, whether the whole string has been
copied.

Intended usage looks like this:

bool fits = string.copy_characters_to_buffer(buffer, sizeof(buffer));

and then either

if (!fits) {
    fprintf(stderr, "The name does not fit!!11");
    return nullptr;
}

or, if you're sure the buffer is large enough,

// I'm totally sure it fits because [reasons go here].
ASSERT(fits);

or if you're feeling extremely adventurous,

(void)fits;

but don't do that, please.

AK/String.h

@@ -146,6 +146,8 @@ public:
     // Includes NUL-terminator, if non-nullptr.
     ALWAYS_INLINE const char* characters() const { return m_impl ? m_impl->characters() : nullptr; }
 
+    [[nodiscard]] bool copy_characters_to_buffer(char* buffer, size_t buffer_size) const;
+
     ALWAYS_INLINE ReadonlyBytes bytes() const { return m_impl ? m_impl->bytes() : nullptr; }
 
     ALWAYS_INLINE const char& operator[](size_t i) const