Add solution for Challenge 6 (#1187)

Co-authored-by: go-interview-practice-bot[bot] <230190823+go-interview-practice-bot[bot]@users.noreply.github.com>

Author: jeffreyyjp

challenge-6/submissions/jeffreyyjp/solution-template.go

@@ -0,0 +1,34 @@
+// Package challenge6 contains the solution for Challenge 6.
+package challenge6
+
+import (
+	// Add any necessary imports here
+	"strings"
+	"unicode"
+)
+
+// CountWordFrequency takes a string containing multiple words and returns
+// a map where each key is a word and the value is the number of times that
+// word appears in the string. The comparison is case-insensitive.
+//
+// Words are defined as sequences of letters and digits.
+// All words are converted to lowercase before counting.
+// All punctuation, spaces, and other non-alphanumeric characters are ignored.
+//
+// For example:
+// Input: "The quick brown fox jumps over the lazy dog."
+// Output: map[string]int{"the": 2, "quick": 1, "brown": 1, "fox": 1, "jumps": 1, "over": 1, "lazy": 1, "dog": 1}
+func CountWordFrequency(text string) map[string]int {
+	// Your implementation here
+	res := make(map[string]int)
+	newText := strings.Replace(text, "'", "", -1)
+	words := strings.FieldsFunc(newText, func(c rune) bool {
+	    return !unicode.IsLetter(c) && !unicode.IsNumber(c)
+	})
+	
+	for _, word := range words {
+	    res[strings.ToLower(word)]++
+	}
+	
+	return res
+}