Parsing for non-root number values fails lazily

(note: same as #105 but for non-root values; related to #679)

Looks like content such as:

```
[ 123true ]
```

or

```
[ 100k ]
```

or

```
[ 100/ ]
```

or

```
[ 1.5x ]
```

passes and reports number token `123` or `100` or `100` or `1.5` (respectively) only failing when trying to access to the following token.
These should be caught earlier.