Improve Coverage for Sylk (#1514)

* Improve Coverage for Sylk

I believe that both BaseReader and Sylk Reader are now 100% covered.

Documentation available for this format is sparse.
It was always incomplete, and in some cases inaccurate.
My goal was to use PhpSpreadsheet to load the test file,
save it as Xlsx, and visually compare the two, then add a test
loaded with assertions. Cell values and calculated values,
and border styles were generally handled pretty well without changes.
Other types of styling were not handled so well. I added a few cells
to exercise some previously uncovered code.

Sylk files must be ASCII. I have deprecated the use of the
setEncoding and getEncoding functions, which had no test cases.

Author: oleibman

samples/templates/SylkTest.slk

@@ -52,7 +52,7 @@ P;EArial;M200
 P;EArial;M200;SI
 P;EArial;M200;SBI
 P;EArial;M200;SBU
-P;EArial;M200;SBIU
+P;EArial;M220;SBIU
 P;EArial;M200
 P;EArial;M200;SI
 F;P0;DG0G8;M255
@@ -115,6 +115,7 @@ F;P19;FG0G;X4
 C;Y7;X2;K2.34
 C;X3;KFALSE
 C;Y8;X2;K3.45
+C;Y9;X2;K2.34;EMEDIAN(R[-3]C:R[-1]C)
 F;Y9;X1
 F;X2
 F;X3