Fix: handle one-var with no semicolon (fixes #10330)

[malcolmsgroves]

What is the purpose of this pull request? (put an "X" next to item)

[ ] Documentation update
[X] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

#10330

What changes did you make? (Give an overview)
The fixer function for one-var can now handle semicolon-less style. The fixer checks if the previous token is actually a semicolon, and if not it just inserts the comma after the previous token instead of replacing it. Should be easy to understand from the edits.

Is there anything you'd like reviewers to focus on?
Based on the existing tests and my understanding of fixer functions, I only changed

var a
var b

to

var a,
 b

instead of

var a, b

Is this the right call?

[not-an-aardvark]

LGTM, thanks for contributing!

[platinumazure]

LGTM, thanks!

Small nitpick: I think the prevSemi ought to be renamed at this point, maybe to something like prevToken. But that shouldn't be a blocker.

[aladdin-add]

LGTM. thanks! Personally I prefer having some more tests for regression:

code: "var i = 0\n j",
code: "var i\n j = 0",
code: "var i = 0\n j = 0",