Replace ~/ with UserHomeDir if the value of synced_folder has the prefix '~/'

[smaruy30]

Description

If one sets the value of the synced_folder as ~/dir, packer responds an error which is shown in #68.
This PR is to solve the error.

Test Results

$ make test
?   	github.com/hashicorp/packer-plugin-vagrant	[no test files]
ok  	github.com/hashicorp/packer-plugin-vagrant/builder/vagrant	1.532s
ok  	github.com/hashicorp/packer-plugin-vagrant/post-processor/vagrant	1.547s
ok  	github.com/hashicorp/packer-plugin-vagrant/post-processor/vagrant-cloud	1.938s
?   	github.com/hashicorp/packer-plugin-vagrant/version	[no test files]

Closes #68

[hashicorp-cla]

All committers have signed the CLA.

[lbajolet-hashicorp]

Hi @smaruy30,

LGTM, thanks for the fix!