Based on standard practices, we need to have a copyright and license notice at the top of each source file. Removed existing copyrights and updated/added copyright notices for .h, .cpp, .cs, .ps1, and .psm1 files.
Updated module manifests for consistency to have Author = "PowerShell" and Company = "Microsoft Corporation". Removed multiple line breaks.
Separate PR coming to update contribution document for new source files: #6140
Manually reviewed each change.
Fix#6073
This reverts the `if !CORECLR ... #endif` guard surrounding FullCLR
files by applying the original diff from the `source-depot` branch, thus
the accidentally overwritten BOMs were restored as well.