Adding of a new exercise printalphabetg.

The goal: printing the alphabet in lowercase from a to g.

go/tests/prog/printalphabetg_prog/main.go

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+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("abcdefg")
+}

go/tests/prog/printalphabetg_test/main.go

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+package main
+
+import (
+	"lib"
+)
+
+func main() {
+	lib.ChallengeMain("printalphabetg")
+}

subjects/printalphabetg/README.md

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+## printalphabetg
+
+### Instructions
+
+Write a program that prints the Latin alphabet until the letter g (included) in lowercase on a single line.
+
+A line is a sequence of characters preceding the [end of line](https://en.wikipedia.org/wiki/Newline) character (`'\n'`).
+
+### Usage
+
+```console
+student@ubuntu:~/[[ROOT]]/printalphabetg$ go build
+student@ubuntu:~/[[ROOT]]/printalphabetg$ ./printalphabetg
+abcdefg
+student@ubuntu:~/[[ROOT]]/printalphabetg$
+```