Expression auto-completion doesn't work in if multi-line expressions

[elbrenn]

Describe the bug
Expression auto-completion doesn't work in if multi-line expressions

Happens only for if where we accept expressions without expression markers.

To Reproduce
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Expected behavior
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Extension Version
0.25.3

Additional context
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[elbrenn]

"This happens because of the transformation code that we run before auto-completion. At that point we don't know if we're in a multi-line string and therefore add : to the end of the line. That interferes with the expression auto-completion code since suddenly it'll see a stray : at the end of the line."