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
```
This PR updates the formatting of, and adds explanations for, "unknown
identifier" errors as well as "failed to infer type" errors for binders
and definitions.
It attempts to ameliorate some of the confusion encountered in #1592 by
modifying the wording of the "header is elaborated before body is
processed" note and adding further discussion and examples of this
behavior in the corresponding error explanation.
