[core] Add TString::size_type member

The `size_type` member is required by the helper macros in the new
CPyCppyy that can be used to declare string converters.

core/base/inc/TString.h

@@ -278,6 +278,7 @@ friend std::strong_ordering operator<=>(const TString &s1, const TString &s2) {
    enum EStripType   { kLeading = 0x1, kTrailing = 0x2, kBoth = 0x3 };
    enum ECaseCompare { kExact, kIgnoreCase };
    static constexpr Ssiz_t kNPOS = ::kNPOS;
+   using size_type = Ssiz_t;
 
    TString();                           // Null string
    explicit TString(Ssiz_t ic);         // Suggested capacity