backslash prefix space in file name

autocomplete on files with spaces looks like `"file\ with\ a\ space.txt"` which doesn't make sense for current grammar (julia).

The issue names no files or tests. Start by reproducing path autocomplete for a filename containing spaces while using the Julia grammar, then trace where the backslash-prefixed suggestion is generated. Done means the behavior is aligned with the current grammar and covered by an appropriate regression check.

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