Given a string s, return the number of homogenous substrings of s. Since the answer may be too large, return it modulo 109 + 7. A string is homogenous if all the characters of the string are the same. A substring is a contiguous sequence of characters within a string. Example 1: Input: s = "abbcccaa" Output: 13 Explanation: The homogenous substrings are listed as below: "a" appears 3 times. "aa" appears 1 time. "b" appears 2 times. "bb" appears 1 time. "c" appears 3 times. "cc" appears 2 times. "ccc" appears 1 time. 3 + 1 + 2 + 1 + 3 + 2 + 1 = 13. Example 2: Input: s = "xy" Output: 2 Explanation: The homogenous substrings are "x" and "y". Example 3: Input: s = "zzzzz" Output: 15 Constraints: 1 <= s.length <= 105 s consists of lowercase letters.