Required Value treated as option

**Synopsis**
If a required value with a `MetaName` is omitted, the error is depicted as Option, which might be misleading:

> A required value not bound to option name is missing.

**Version used**

2.8.0

**System Description**
- Windows 10 (.NET FullFramework 4.8)
- Ubuntu Linux 18.04 (.NET Core 3.1)
- Target Assembly Compiled as: .NET Standard 2.0

---

**Gist for reproduction**
[Example Repository](https://gist.github.com/742d370b56766aaa5fec64959aae6f03.git)

In this gist, two verbs are introduced both with a required positional argument. Both Values have a MetaName. The meta name is used in the description during the help text, but not in the error message.

---

**Actual behavior**

```
$ calc fib --help
calc 1.0.0
Copyright (C) 2021 calc

--help Display this help screen.

--version Display version information.

position of the fibonacci number to calculate (pos. 0) Required.
```

This is ok and expected. But omitting the `--help` leads to the following error:

```
$ calc fib
calc 1.0.0
Copyright (C) 2021 calc

ERROR(S):
A required value not bound to option name is missing.

--help Display this help screen.

--version Display version information.

position of the fibonacci number to calculate (pos. 0) Required.

```

**Expected behavior**

```
$ calc fib
calc 1.0.0
Copyright (C) 2021 calc

ERROR(S):
The required position of the fibonacci number to calculate (value at position 0) is missing.

--help Display this help screen.

--version Display version information.

position of the fibonacci number to calculate (pos. 0) Required.
```

---

**Possible error sources analyzed**

In [SpecificationPropertyRules](https://github.com/commandlineparser/commandline/blob/24e2be23ce695d36316d921d2a52f9eb94cb0558/src/CommandLine/Core/SpecificationPropertyRules.cs#L149), a `MissingRequiredOptionError` error is issued for the ValueSpecification using the [NameExtensions `FromSpecification` method](https://github.com/commandlineparser/commandline/blob/24e2be23ce695d36316d921d2a52f9eb94cb0558/src/CommandLine/Core/NameExtensions.cs#L7-L30). For a ValueSpecification, the resulting `NameInfo` object is set to an empty instance. This might be intentional, but will cause the error here.

---

**Suggested fixes**

1. Extend `NameExtensions` class as follows:

```
public static NameInfo FromValueSpecification(this ValueSpecification specification)
{
return !string.IsNullOrWhitespace(specification.MetaName)
? new NameInfo(
string.Empty,
specification.MetaName)
: NameInfo.EmptyName ;
}

public static NameInfo FromSpecification(this Specification specification)
{
switch (specification.Tag)
{
case SpecificationType.Option:
return FromOptionSpecification((OptionSpecification)specification);
case SpecificationType.Value:
return FromValueSpecification((ValueSpecification)specification);
default:
return NameInfo.EmptyName;
}
}
```

This clarifies the error message more to
```
ERROR(S):
​Required option 'position of the fibonacci number to calculate' is missing.
```

2. If this is not yet appropriate enough, we can update [SpecificationPropertyRules](https://github.com/commandlineparser/commandline/blob/24e2be23ce695d36316d921d2a52f9eb94cb0558/src/CommandLine/Core/SpecificationPropertyRules.cs#L149) in **addition to 1** like follows:

```
from sp in missing
select
sp.IsOption() ? new MissingRequiredOptionError(sp.FromSpecification()) as Error
: new MissingValueOptionError(sp.FromSpecification()) as Error;
```

This changes the error to
```
Option 'position of the fibonacci number to calculate' has no value.
```

which might not be good.

3. Take approach (2) but introduce a new `MissingRequiredPositionalValueError` including the extension of Errors, SentenceBuilder, etc.

If you accept the issue, I can submit a PullRequest for each of the three issues, if you tell me, which way to go.

If you do not accept the issue, please tell me a feasible workaround or tell me the reason, why this should be considered as intentional.

Cheers

Carsten