[lldb-vscode] Use `expression` command for completion

Change lldb-vscode to use the `expression` command for generating completions, instead
of the `p` alias.

Aliases are user overrideable, and even deletable, but the `expression` command is
unchangeable.

See D141539 where a similar replacement was done to tests.

Differential Revision: https://reviews.llvm.org/D145437