Check: Invalid date format

On columns that specify a `datetime` or `date`, warn of an invalid date. We should also do this on date and time functions,

Sample output:

```
Invalid date '45/67/2018' specified for column 'purchase_date'
```

```
Invalid date '45/67/2018' given for call to 'DATEDIFF'
```

See the list here:
https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html

- [x] - Create a new directory `src/syntax`. The `syntax` directory should be responsible for holding all keywords, functions, operators etc... of the language
- [x] - Move `Keywords.ts` into this directory
- [x] - Create a new directory `src/syntax/{driver}/functions/datetime/`
- [x] - Create a file in here `src/syntax/{driver}/functions/datetime/functions.ts`

Note the `{driver}` part of the path. This can be any valid driver supported by `sql-lint`, for now, just do `mysql`.