Fix Out/Error streams chunking in Fsi tool window

[OmarTawfik]

Fixes #864
The issue is that chunking the output streams (standard output and error) was done in order:

Index	Type	Line
1	error	System.DivideByZeroException: Attempted to divide by zero.
2	out	>
3	error	at <StartupCode$FSI_0077>.$FSI_0077.main@()
4	error	Stopped due to error

Per the previous chunking function, this produced chunks of [(1), (2), (3,4)], and the following output (notice that the prompt is after the first line):

System.DivideByZeroException: Attempted to divide by zero.
>   at <StartupCode$FSI_0077>.$FSI_0077.main@()
Stopped due to error

The new chunking function groups across lines, producing chunks of [(1,3,4), (2)], and the following output (the prompt is now separate from error lines):

System.DivideByZeroException: Attempted to divide by zero.
    at <StartupCode$FSI_0077>.$FSI_0077.main@()
Stopped due to error
>

@dsyme @abelbraaksma @Microsoft/fsharp-compiler

[KevinRansom]

is it rec?

[OmarTawfik]

@KevinRansom no. Refactoring miss. I'll change it now. Thanks for noticing.

[enricosada]

@dsyme maybe that was the issue with err/out stream redirection on test suite?