Issue 13538 & 13539 - Improve variable declaration grammar for old C style syntax

[Issue 13538](https://issues.dlang.org/show_bug.cgi?id=13538) - Divide old C style syntax from D style grammar rule
[Issue 13539](https://issues.dlang.org/show_bug.cgi?id=13539) - Disallow optional template parameters with C-style syntax