Use dashes in options -- not underscores

[kasperl]

It seems much more conventional to use dashes in options instead of underscores. I suggest we use:

  dart --enable-checked-mode

rather than:

  dart --enable_checked_mode

Alternatively, we can support both (like V8 does) but I'd prefer not supporting underscores altogether.

[ghost]

VM already supports both dashes and underbars in flags (except for some very special flags, e.g., --break_at, --compile_all, --generate_pprof_symbols, --use_script_snapshot, --import_map).
For all conventional flags (including soon-to-be-addded "enable-checked-mode") we have decided to support both, at least for the moment.
Maybe we should print flags with dashes instead of underbars (--print-flags).

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cc @iposva-google.
Set owner to @sgmitrovic.
Added Accepted label.

[iposva-google]

The canonical version inside the VM is with underscores. This makes it easy to copy and paste the relevant flag to a command-line. Since it is not possible to name the flags with dashes because that conflicts with the - operator in C++, I guess this should stay. I verified that the form --some_flag and --some-flag are equivalent and enabled the proper handling of --no_some_flag and --no-some-flag. So we should be covered for pretty much all of it.

VM development flags handled by the binary, such as --compile_all are the lone exception.

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Set owner to @iposva-google.
Added Fixed label.