Changes:
- Adds two .reg scripts:
1. install-context.reg
adds context-menu entries for 'Git GUI Here' and 'Git Bash Here'
2. install-file-associations.reg
associates .git* files as plain-text, and associates .sh with Bash
Notes:
- Closes: #4462
- The file-naming convention and post-install script were based on the
`7zip.json`, `vim.json` and `pwsh.json` manifest files.
- The registry keys set by the .reg files are identical to those set by
`[install.iss][1]` in the `[git-for-windows/build-extra][2]` repo.
[1]: <https://github.com/git-for-windows/build-extra/blobl/main/installer/install.iss>
[2]: <https://github.com/git-for-windows/build-extra>
