New-MarkdownHelp - add info-string to fenced code blocks

[johkoenig]

Steps to reproduce

This is loosely connected to #318. It would be nice to have "powershell" added to the fenced code boxes. This applies to syntax and examples.

Expected behavior

## SYNTAX
```powershell
New-DemoFunction [<CommonParameters>]
```

## EXAMPLES

### EXAMPLE 1
```powershell
C:\> New-DemoFunction
```

Actual behavior

## SYNTAX
```
New-DemoFunction [<CommonParameters>]
```

## EXAMPLES

### EXAMPLE 1
```
C:\> New-DemoFunction
```

Environment data

v0.12.0

[vors]

In #294 we settled on adding the language moniker to examples.
For a long explanation about why I don't think it's a good idea for SYNTAX, see #294 (comment)

[vors]

Closing in favor of #294