improve spelling heuristic

```
% starlark
Welcome to Starlark (go.starlark.net)
>>>
>>>
>>> def f(js_library, AsLibrary):
... _ = as_library
...
:2:7: undefined: as_library (did you mean AsLibrary?)
```

The spell checker ignores case and underscores, but perhaps it shouldn't, or perhaps it should ignore them only if it go no matches when considering them. (Perhaps that can be implemented cleanly by treating the distance from 'a' to 'A' as less then 1.0.)

Reproduce the Starlark REPL example and inspect the spell-checker implementation, which is the only component identified by the issue. Compare suggestions when case and underscores are preserved versus ignored, then verify that the undefined as_library example no longer prefers AsLibrary while existing spelling suggestions still work.

Written by the indexing model from the issue text.