Narrow explode() result for limit 0, 1, and -1

[paulbalandan]

Follow-up to #5959, refining explode()'s return type for small limits, per the explode() manual:

• Limit 0 or 1 returns the whole string unsplit (a limit of 0 is treated as 1), so the result is the single-element shape array{string} instead of non-empty-list<string>. This holds regardless of whether the delimiter is present.
• Limit -1 with a delimiter proven present drops only the last of the (at least two) parts, leaving at least one, so non-empty-list<string> instead of list<string>. Limit -2 and beyond can still empty the result and stay list<string>.

[VincentLanglet]

We need assertion on

explode("\n", $s, -1)

without the str_contains check

[paulbalandan]

Added: explode("\n", $s, -1) without the str_contains guard now asserts list<string>.

[VincentLanglet]

Thanks