Remove UTF-8 BOM from text files

[xtqqczze]

PR Summary

• Remove UTF-8 BOM from text files.
• Specify utf-8 charset in .editorconfig to prevent IDEs inserting BOM.

PR Checklist

• [x] PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• [x] Summarized changes
• [x] Change is not breaking
• [x] Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
• [x] Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
• [x] This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: to the beginning of the title and remove the prefix when the PR is ready.

[rjmholt]

Remove UTF-8 BOM from text files.

My main concern is with this part; Windows PowerShell isn't going to like BOM-less UTF-8

[rjmholt]

Instead, ps*1 files should probably have a BOM and other text files should not

[rjmholt]

The configuration should match that, but generally looks good to me

[rjmholt]

@xtqqczze after looking through your approach in this PR and thinking about some of the particulars of this repo, I've opened https://github.com/PowerShell/PSScriptAnalyzer/pull/1665, which goes a bit further and tries to clean up some of the strangeness in the PSSA repo.

In particular, it specifies more settings, uses UTF-8 with BOM for PowerShell files, and makes sure test assets (some of which are deliberately encoded or formatted differently) are put in a protected assets directory

[xtqqczze]

@rjmholt I will close once #1665 is merged.