LibCore: Let IODevice::can_read_line() buffer until \n or EOF

If a line was larger than 1024 bytes or the file ended without a
newline character, can_read_line would return false.

IODevice::can_read_line() now reads until a newline is found or
EOF is reached.

fixes #5907

AK/Vector.h

@@ -161,8 +161,6 @@ public:
         return false;
     }
 
-    // NOTE: Vector::is_null() exists for the benefit of String::copy().
-    bool is_null() const { return false; }
     bool is_empty() const { return size() == 0; }
     ALWAYS_INLINE size_t size() const { return m_size; }
     size_t capacity() const { return m_capacity; }