Function countSubstring

countSubstring(str: string, sub: string): number
Counts how many times a substring occurs within a string.
Parameters
- str: string
  The input string.
- sub: string
  The substring to count.
Returns number
Number of times sub occurs in str.
Remarks
- Overlapping substrings are not double-counted.
- Returns 0 if the substring is not found.
Example
```
countSubstring("lololol", "lo"); // 3
countSubstring("aaaaa", "a");    // 5
countSubstring("hello", "z");    // 0
```
- Defined in src/string/fn.ts:98