Buffer writes in blocks of 4Kb when writing buffers to files

Could make kakoune more compatible with tools looking for file
modifications by reducing the amount of writes done.

As discussed in #2812

src/file.cc

@@ -257,6 +257,41 @@ void write(int fd, StringView data)
     }
 }
 
+struct BufferedWriter
+{
+    BufferedWriter(int fd) : fd{fd} {}
+
+    ~BufferedWriter()
+    {
+        if (pos != 0)
+            Kakoune::write(fd, {buffer, pos});
+    }
+
+    void write(StringView data)
+    {
+        while (not data.empty())
+        {
+            const ByteCount length = data.length();
+            const ByteCount write_len = std::min(length, size - pos);
+            memcpy(buffer + (int)pos, data.data(), (int)write_len);
+            pos += write_len;
+            if (pos == size)
+            {
+                Kakoune::write(fd, {buffer, size});
+                pos = 0;
+            }
+            data = data.substr(write_len);
+        }
+    }
+
+private:
+    static constexpr ByteCount size = 4096;
+    int fd;
+    ByteCount pos = 0;
+    char buffer[(int)size];
+};
+
+
 void write_buffer_to_fd(Buffer& buffer, int fd)
 {
     auto eolformat = buffer.options()["eolformat"].get<EolFormat>();
@@ -266,17 +301,18 @@ void write_buffer_to_fd(Buffer& buffer, int fd)
     else
         eoldata = "\n";
 
+
+    BufferedWriter writer{fd};
     if (buffer.options()["BOM"].get<ByteOrderMark>() == ByteOrderMark::Utf8)
-        if (::write(fd, "\xEF\xBB\xBF", 3) < 0)
-            throw runtime_error(format("unable to write data to the buffer (fd: {}; errno: {})", fd, ::strerror(errno)));
+        writer.write("\xEF\xBB\xBF");
 
     for (LineCount i = 0; i < buffer.line_count(); ++i)
     {
         // end of lines are written according to eolformat but always
         // stored as \n
         StringView linedata = buffer[i];
-        write(fd, linedata.substr(0, linedata.length()-1));
-        write(fd, eoldata);
+        writer.write(linedata.substr(0, linedata.length()-1));
+        writer.write(eoldata);
     }
 }