Add completion possibilities that split the current token

Consider the following (where the vertical line represents the cursor position):

program = dr|codeWorldLogo

Triggering autocomplete will now try to match the entire token, `drcodeWorldLogo`. Autocomplete will decide that `drawingOf` is the best suggestion by far, and turn the whole thing into `drawingOf`. The `codeWorldLogo` is lost.

Instead, auto-complete should be willing to also consider splitting the token into `dr` (before the cursor) and `codeWorldLogo` at the end. If they are BOTH close to existing tokens, then autocomplete should potentially replace the `dr` with `drawingOf`, and leave `codeWorldLogo` alone. The decision about whether to split the current token or not should be made based on the edit distance to turn the portion after the cursor into a known token, as compared to the edit cost of deleting it after this completion, possibly with some weighting in favor of one or the other choice.