fix(splitter): improve EOF handling

[juleswritescode]

Currently, if you type select case in a file, the LSP will crash.

The reason is that we forgot to handle the EOF token in the case() function in the splitter.

I feel that, since we deal with incomplete statements all the time, it's better to bubble up an Err if we reach EOF:

• it's an unrecoverable error and should immediately stop splitting
• this way, we'll never forget adding the EOF to any statement again

But it's a bit invasive, so WDYT?

(Pretty sure this fixes #540, it's the same kind of error)

[juleswritescode]

note: we do not need to handle EOF below anymore

[psteinroe]

I like it! maybe we could add our own Result type that always returns the eof error to make sure we don't accidentally bubble up another error and handle it in the wrong way?