change splitTextIntoLines() to splitLines()

[WalterBright]

Cribbing code from std.string.splitLines():

1. returns an InputRange rather than an array of lines
2. no longer does any memory allocation (yay!)
3. no more leaking memory
4. stops processing the source text when the desired line is found
5. supports Unicode line endings (fixing unnoticed benign bug)
6. uses Voldemort return value
7. is pure, @safe, @nogc and all that good stuff
8. much faster
9. completely self-contained

[dlang-bot]

Thanks for your pull request, @WalterBright!

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#15972"

[WalterBright]

Looks like our respite from test suite heisenbugs was, alas, too brief.