Optimize writing to StreamWriter

[apetukhov]

writing the formatted string directly to the StreamWriter, using an existing method that has the same semantics as string.Format
fix .cs files encoding to UTF-8 with BOM
fix used culture in TestCellFormat tests

[tonyqus]

Since this is a performance optimzation PR, let's go further.

Direct Write (Fastest): Using streamWriter.Write(string) sends text directly to the internal buffer, requiring minimal CPU overhead.
Formatted Write (Slower): Using streamWriter.Write("Value: {0}", value) forces the system to parse the string, perform string conversion, and then write, increasing CPU time.

How about getting rid of the formatted write so that it can be faster?

For example

        sw.Write("<c:{0}", nodeName);

To

        sw.Write("<c:");
        sw.Write(nodeName);

[tonyqus]

In #1095, it has some similar optimization

sw.Write(string.Format("</w:{0}>", nodeName));

To

sw.WriteEndW(nodeName);

And

sw.Write(string.Format("</{0}>", nodeName));

To

sw.WriteEndElement(nodeName);

Please run the following beachmarks to see if there is real optimization as #1095 did.

NPOI.Benchmarks.LargeExcelFileBenchmark
NPOI.Benchmarks.RangeValuesBenchmark

[tonyqus]

@lahma Do you have any further suggestion?

[lahma]

agree with removing the format altogether, should be easy regex or AI work, also instead of sw.Write(">"); write using the char overload sw.Write('>');