FileRegionsCache multiline context region generation needs a look.

[michaelcfanning]

the FileRegionsCache multiline context generation is pretty idiosyncratic. Some considerations:

• The code that executes is conditional depending on whether the input region is driven by startLine or charOffset conventions. It would be useful to minimize testable surface area if we can by simplifying.
• There is a distinction between minified/non-minified code.
• There appear to be some distinctions depending on the size of the input context region itself (i.e., is it greater/lesser than the large snippet size of 512 bytes.

The behavior of the relevant api ConstructMultilineContextSnippet needs to be clearly specified. Tests need to be updated to reflect and test expecations.