fix(install): add DTVEM_ROOT support to PowerShell installer #175
Description
The PowerShell install script (install.ps1) does not respect the DTVEM_ROOT environment variable, unlike the bash script (install.sh).
The website documentation at https://dtvem.io/docs/user-guide/getting-started tells users they can set DTVEM_ROOT before installing to use a custom location, but this only works on macOS/Linux.
Expected behavior:
Setting $env:DTVEM_ROOT before running the installer should install to that custom directory.
Actual behavior:
The PowerShell script ignores DTVEM_ROOT and always installs to %USERPROFILE%\.dtvem.