Do not report MA0028 for concatenation of constant strings and chars

[meziantou]

Fixes #1528

MA0028 reported sb.Append(" " + Const + " ") where Const is a const char, although the compiler merges this concatenation into a single string. The char operand is an implicit conversion rather than a constant string, and the left side is itself a nested concatenation, so the rule did not recognize it as constant.

Changes:

• IsConstString now also accepts an implicit conversion of a constant char, and a string + whose operands are all constant (recursively).
• Concatenations that include a non-constant value (e.g. "a" + 'b' + 10) are still reported.
• Added tests and a note in docs/Rules/MA0028.md.

MA0028 tests pass on Roslyn 5.9 and 4.8.