cores/WString: Add missing __FlashStringHelper implementations

[ederjc]

Problem

Using the F() macro with String operations on XMC boards (V3.x) causes linker errors:

undefined reference to `String::String(__FlashStringHelper const*)'
undefined reference to `String::operator=(__FlashStringHelper const*)'
undefined reference to `String::concat(__FlashStringHelper const*)'
undefined reference to `operator+(StringSumHelper const&, __FlashStringHelper const*)'

WString.h declares 5 __FlashStringHelper overloads but WString.cpp was missing all their implementations. This affects any library that uses F() with String (e.g. SimpleSerialShell).

Root Cause

The V3.x branch uses its own cores/WString.h/.cpp (unlike V4.x which uses ArduinoCore-API). The header correctly declared the 5 methods, but the implementations were never added to WString.cpp.

Fix

Added the 5 missing implementations to cores/WString.cpp:

• String::String(const __FlashStringHelper*)
• String::copy(const __FlashStringHelper*, unsigned int)
• String::operator=(const __FlashStringHelper*)
• String::concat(const __FlashStringHelper*)
• operator+(const StringSumHelper&, const __FlashStringHelper*)

On XMC (non-AVR), pgmspace.h maps strcpy_P/strlen_P to plain strcpy/strlen, so the implementations are functionally identical to the existing const char* variants.

Testing

Reproduced the 4 linker errors with the stock 3.5.2 install, then confirmed clean compilation after the fix using arduino-cli with Infineon:xmc:XMC1100_Boot_Kit.

Fixes #439

[jaenrig-ifx]

I won't review much in detail. This seems to be available already in the coreAPI -> https://github.com/arduino/ArduinoCore-API/blob/master/api/String.cpp
Therefore this should not be an issue for 4.x ...