Berry int64.fromstring() to convert a string to an int64

[s-hadinger]

Description:

Add to int64 a class method to convert from a string to an int64.

print(9876543210)
1286608618     <- wrong because the number overflows 32 bits

print(int64.fromstring("9876543210"))
9876543210

print(int64.fromstring("9876543210") * int64(1000))
9876543210000

Note: int64 requires #define USE_BERRY_INT64

Checklist:

• The pull request is done against the latest development branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works with Tasmota core ESP8266 V.2.7.4.9
• The code change is tested and works with Tasmota core ESP32 V.2.0.6
• I accept the CLA.

NOTE: The code change must pass CI tests. Your PR cannot be merged unless tests pass