keywords scoped in places where not actually keywords, IF inside simple interpolation `()` for instance

Start by reproducing the reported scoping with the PowerShell samples in the issue and compare the result with PSReadLine, where the construct is treated as a command. Inspect the syntax grammar's root patterns and repositories, then verify that if and similarly limited keywords such as in are scoped only where valid, without regressing valid interpolation highlighting.

Environment

• Editor and Version: VS Code: 1.31 Insiders / 1.30.1
• Your primary theme: Monokai Dimmed (modified)

Issue Description

In the following image and code, the if is being scoped as 'keyword.control', as if it was valid to utilize an IF statement at this point. However it is not valid, and the code instead causes an error stating that The term 'if' is not recognized as the name … No syntax error is reported by EditorServices either. PSReadLine however does indicate that the if would be treated as a command instead of a keyword.

Screenshots

VS Code:

PSReadLine

(keywords would be green)

Expected Behavior

I would like to expect that this case would not receive 'keyword.control' scoping, so that I am not led to believe the given command might actually be correct. A quick attempt to correct this indicates that this will require some reorganization, because too many items are in the root patterns and not in the repository.

This also shows that there is more to argument mode vs. expression mode than I had originally thought. Control statements cannot always be used everywhere a command can be used. This also affects other limited scope keywords, such as in, which is scoped as 'keyword.control' in more places than it should be.

Code Samples

    '<?xml version="1.0"' + (if ($encoding) {' encoding="' + $encoding + '"'}) + '?>'

The correct code:

    '<?xml version="1.0"' + $(if ($encoding) {' encoding="' + $encoding + '"'}) + '?>'