gopls/internal/golang: fix folding range for function calls

[ShoshinNikita]

Make folding ranges for function calls consistent with folding
ranges for function bodies and composite literals:

• keep the closing parenthesis visible
• do not generate a folding range if either the first or last
  argument is on the same line as the opening or closing parenthesis

Also refactor some code, add additional comments and test cases.

Fixes golang/go#70467

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[gopherbot]

Message from Robert Findley:

Patch Set 1:

(4 comments)

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[gopherbot]

This PR (HEAD: 1cf77e8) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/tools/+/630056.

Important tips:

• Don't comment on this PR. All discussion takes place in Gerrit.
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[gopherbot]

Message from Nikita Shoshin:

Patch Set 2:

(3 comments)

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