Expand extractSymbol ranges

[jessetrinity]

Fixes #41469

When a refactor is invoked through a keyboard command we search a bit harder for the extract symbol refactor by treating the selected span as though it covers the tokens containing the start and end tokens.

That is, selecting

const foo = ba[|r + b|]az;

will be equivalent to selecting

const foo = [|bar + baz|];

for the purposes of offering the refactor to extract bar + baz.

I also separated out the unit tests into individual tests so we can tell them apart on failure.

[jessetrinity]

cc @mjbvz

[amcasey]

Does this resolve #41469 or is it just related? That issue seems to be about doing this for all refactorings.

[amcasey]

Mostly questions.

[jessetrinity]

I expect that #41469 only relates to extractSymbol. Other refactorings generally only use the start of the span to search for an applicable refactor, so the "explicit cursor request" from #38378 should suffice. extractSymbol relies more on the start and end nodes both being in the right places so it needed special logic to "try harder".

[amcasey]

Thanks!