Suggestion: convert sosplugin commands to sub-commands of `sos` command

Currently, some SOS commands are ambiguous and slightly confusing from UX viewpoint. An example is `registers` command with alias `r`; in lldb, `r` is reserved for `run` command so this alias does not get overridden when sosplugin is loaded. Also, `register` (singular) is a built-in command so users have to remember to pluralize if they intentionally wanted to execute the one out of sosplugin.

It would be nice if all SOS command are prefixed with `sos`, so instead of:

```gdb
(lldb) soshelp
(lldb) clrstack -f
(lldb) lm
(lldb) registers
# and so on
```

we could have:

```gdb
(lldb) sos help
(lldb) sos clrstack -f
(lldb) sos lm
(lldb) sos registers
# and so on
```