Improve patching-under-Windows compat

[Finii]

Description

Improve Windows as patching OS compatibility by

• allowing absolute path to begin with a drive letter
• assembling path with the system path separator

And finally, for all systems:

• Specify encoding of glyphnames datafile to avoid surprises

Requirements / Checklist

• Read the Contributing Guidelines
• I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan.
  Issue number where discussion took place: #xxx
• If this contains a font/glyph add its origin as background info below (e.g. URL)
• Verified the license of any newly added font, glyph, or glyph set. License is: xxx

What does this Pull Request (PR) do?

Apart from other stuff, allow a drive letter in the outputdir parameter:

Example:

PS C:\Program Files (x86)\FontForgeBuilds\bin> .\fontforge.exe
    --script C:\Users\jastr\Downloads\FontPatcher\font-patcher
    --outputdir c:\Users\jastr
    C:\Users\jastr\Downloads\Miosevka12.\Font\Miosevka12\Miosevka-Regular.ttf

How should this be manually tested?

Any background context you can provide?

What are the relevant tickets (if any)?

• Not able to open glyphnames json when running patcher 3.3.0 script in WIndows #1761

Screenshots (if appropriate or helpful)