Deprecate support for `tns init` command

[rosen-vladimirov]

Is your feature request related to a problem? Please describe.
tns init command will be deleted in NativeScript 6.0.0 as it doesn't make any sense to be used now. The idea of command was to support migration of non-NativeScript project to NativeScript one. Now, as we have schematics, the command lost its meaning.

Describe the solution you'd like
Show deprecation message when tns init command is executed.

Describe alternatives you've considered
None

Additional context
The command should be executed in non-NativeScript project, i.e.:

$ mkdir test1
$ cd test1
$ npm init -f
$ tns init