VIM-2615 add support to sort u command

[samabcde]

1. support sort u
2. fix natural sort issue when both string not contain number
3. refactored SortCommandTest

[lippfi]

Thank you for this PR!

[AlexPl292]

Hi there, thank you for your contribution.
I'm not sure, will it work fine with the i flag and two lines with the same word of different cases? I guess, distinct won't eliminate such lines, will it?
I've tried to find this case in test cases, but looks like it's not tested. Something like:

content = """
data
DATA
""",
command = "sort ui",
expected = "data",

[samabcde]

Hi there, thank you for your contribution. I'm not sure, will it work fine with the i flag and two lines with the same word of different cases? I guess, distinct won't eliminate such lines, will it? I've tried to find this case in test cases, but looks like it's not tested. Something like:

content = """
data
DATA
""",
command = "sort ui",
expected = "data",

@AlexPl292 Thx for pointing out. Yes, you are correct that distinct won't eliminate such lines.
I have updated the PR:

1. Implemented unique with case insensitive handling in ChangeGroup#sortTextRange
2. Corresponding tests are SortCommandTest#caseInsensitiveUniqueSortTestCases and SortCommandTest#numericCaseInsensitiveReverseUniqueSortTestCases
   Please review again.

[AlexPl292]

Okay, this seems to be a better solution. Glad to see this supported, thank you for your PR!