According to the language spec, a struct field name should
be an identifier.
identifier = letter { letter | unicode_digit } .
letter = unicode_letter | "_" .
Implements a function 'isValidFieldName(fieldName string) bool'.
To check if the field name is a valid identifier or not.
It will panic if the field name is invalid.
It uses the non-exported function implementation 'isLetter'
from the package 'scanner', used to parse an identifier.