target-async, an alias for mi-async, is deprecated

[GitMensch]

When using GDB 11.1 (over ssh but this should not matter) the user is prompted with a deprecation message:

Warning: 'set target-async', an alias for the command 'set mi-async', is deprecated.
Use 'set mi-async'.

It may be reasonable to either query the GDB version or execute that depending on a launch configuration option.

[GitMensch]

Just rechecked for the Vim TermDebug plugin which used both "to ensure everything is fine" - set mi-async is a GDB 7.8 feature (Released Jul 2014), which also comes with -gdb-set mi-async option. @WebFreak001: Do you know what GDB version this extension currently needs? The minimal supported versions of GDB/LLDB are missing in README.md.

What do you think of:

• query the version after start, store in the mi class and make the code depending on that
• try the new command - if there's an error use the old one
• always use the new one, effectively making GDB 7.8 the minimal version (if there isn't anything else that requires a newer version)

[GitMensch]

So some months passed. I'd actually suggest to:

• implement autorunBefore from #99
• in the new default use gdb-set mi-async (users with old GDB version can adjust this)