[ruby-core:91256] [Ruby trunk Feature#15562] `String#split` option to suppress the initial empty substring

Issue #15562 has been reported by sawa (Tsuyoshi Sawada).

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Feature #15562: `String#split` option to suppress the initial empty substring
https://bugs.ruby-lang.org/issues/15562

* Author: sawa (Tsuyoshi Sawada)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
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`String#split` returns an empty substring if any at the beginning of the original string, even though it does not return an empty substring at the end of the original string:

```ruby
"aba".split("a") # => ["", "b"]
```

This is probably heritage from Perl or AWK, and may have some use cases, but in some (if not most) use cases, this looks asymmetric, and the initial empty string is unnatural. I propose to give an option to `String#split` to suppress it, perhaps like this (with `true` being the default):

```ruby
"aba".split("a", initial_empty_string: false) # => ["b"]
"aba".split("a", initial_empty_string: true) # => ["", "b"]
"aba".split("ba", initial_empty_string: true) # => ["b"]
```

This does not mean to suppress empty strings in the middle. So it should work like this:

```ruby
"aaaba".split("a", initial_empty_string: false) # => ["", "", "b"]
"aaaba".split("a", initial_empty_string: true) # => ["", "", "", "b"]
```

Or may be we can even go on further to control both the initial and the final ones like (with `:initial` being the default):

```ruby
"aba".split("a", terminal_empty_string: :none) # => ["b"]
"aba".split("a", terminal_empty_string: :initial) # => ["", "b"]
"aba".split("a", terminal_empty_string: :final) # => ["b", ""]
"aba".split("a", terminal_empty_string: :both) # => ["", "b", ""]
```




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