Protect certain frontmatter metadata fields

[sdwheeler]

Summary of the new feature / enhancement

There are 2 metadata fields that should not be allowed to be overwritten by values passed to cmdlet using the Metadata parameters:

• PlatyPS schema version
• document type

Proposed technical implementation details (optional)

• The value of those fields should reflect what the output methods are creating
• If the user includes those fields in the hashtable passed to the Metadata parameter, the values ignored and a warning written to tell the user.

[sdwheeler]

In https://github.com/PowerShell/platyPS/commit/27d5ffe4090f146b64c7d91453d6683c68c8ee53

Tests

• Export-MarkdownModuleFile - fixed and writes warnings about trying overwrite keys
• Export-YamlModuleFile - FAIL - keys overwritten
• Export-MarkdownCommandHelp - keys are not overwritten, but no warning messages about attempt to overwrite
• Export-YamlCommandHelp - keys are not overwritten, but no warning messages about attempt to overwrite