Issue with insertSep function in strutils
### Nim Version

insertSep() misbehaves when not dealing with numeric strings.

Not an earth shattering issue since it is easy enough to recode what insertSep does but if the function is there someone might like to look at why it does not behave as I would expect.

See example below.

### Description

```
#!/usr/bin/env -S nim r
# File: test4.nim
# Nim Compiler Version 2.2.2 [Linux: amd64]

import std/strutils

# insertSep documentation suggests that the function is primarily useful for
# splitting numbers but it does not say it will fail if it is used otherwise

# The following works as expected
let src1 = "1234567890123456"
# The line prints "1234 5678 9012 3456"
echo src1.insertSep( ' ', 4)


# The following does not work as expected
# It includes 2 seperators but does not add the third.
let src2 = "SGVsbG8gV29ybGQ="
# The line prints "SGVsbG8g V29y bGQ=" but I would expect "SGVs bG8g V29y bGQ="
echo src2.insertSep( ' ', 4)


```

### Current Output

```text

```

### Expected Output

```text

```

### Known Workarounds

Just rewrote what I needed without using insertSep

### Additional Information

_No response_