amend cursor position decrement logic & create language service command kernel test

[cartercanedy]

Changes:

• Refactored the logic around cursor position decrement to skip conditionals altogether if cursor position is at pos 0
• Added a test to catch a kernel crash on submission of a language service command where the cursor position is set to 0 in a 0-length buffer

[colombod]

Thank you for this contribution

[shyamnamboodiripad]

Thanks for the contribution @cartercanedy! Could you please clarify why the test was excluded from netfx? I see that some other tests in the file were already excluded - but I don't remember why.

[cartercanedy]

It looks like the test case is available for both netcore and netfx through Microsoft.DotNet.Interactive.Tests Microsoft.DotNet.Interactive.VisualStudio.Tests. The test uses a CSharpKernel nested in a CompositeKernel, which is unavailable in the latter.

[shyamnamboodiripad]

Ah indeed - that makes sense. I was wondering if there was some other reason why the test fails on netfx. Thanks for clarifying.