Use adjusted completion position when at end of identifier

[DanielRosenwasser]

Fixes #2483.

Context

When requesting completion off of something that's not a dotted name, we need to traverse the scopes. First we need to grab a node for which we will traverse upwards to aggregate scopes. We call this the scopeNode.

The original idea was that the first node that we begin aggregating on needs to contain the position for which we are requesting completion. The original challenge was determining "containedness". Fundamentally, a node n contains a position p if n.getStart() <= p && p < n.getEnd() _or_ the n.getStart() <= p and n is not "completed". See #2396 for details.

However, when requesting completion at the end of an identifier, this is an issue because the identifier is completed. This surfaces in issues with arrow functions like in the following:

xyz => xy/*$*/

Here, the arrow function is considered completed, so we continue looking for a node that owns the position of the cursor.

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Solution

In the same manner that we adjust the contextToken, we adjust the position for which we are requesting a completion if at the end of an identifier. For the previous example, we will find scopes as if the completion was requested in the following scenario:

xyz => /*$*/xy

[CyrusNajmabadi]

Note: currentToken may also be a keyword. i.e. you wrote "of" in the process of typing 'often'. So you don't want to check for just identifier, but also for keyword. In roslyn, we just call these "Words".