This PR changes the way that scientific numerals are parsed in order to
give better error messages for (invalid) syntax like `32.succ`.
Example:
```lean4
#check 32.succ
```
Before, the error message is:
```
unexpected identifier; expected command
```
This is because `32.` parses as a complete float, and `#check 32.`
parses as a complete command, so `succ` is being read as the start of a
new command.
With this change, the error message will move from the `succ` token to
the `32` token (which isn't totally ideal from my perspective) but gives
a less misleading error message and corresponding suggestion:
```
unexpected identifier after decimal point; consider parenthesizing the number
```
