+ ruby-[parse, rewrite]: add legacy switches

[marcandre]

This PR adds switches to ruby-parse and ruby-rewrite to turn off emit_lambda, etc.

I thought that --legacy would be an easy to remember switch and also added --legacy-lambda, etc. just because.

Usecase: The actual AST printed / passed on to rewriter is quite important, so these switches should be available. In particular, rubocop currently does not turn them on, so writing/debugging cops using ruby-parse is much more error prone.

New help look like:

$ ruby-parse -h
Usage: ruby-parse [options] FILE|DIRECTORY...
        --18                         Parse as Ruby 1.8.7 would
        --19                         Parse as Ruby 1.9.3 would
        --20                         Parse as Ruby 2.0 would
        --21                         Parse as Ruby 2.1 would
        --22                         Parse as Ruby 2.2 would
        --23                         Parse as Ruby 2.3 would
        --24                         Parse as Ruby 2.4 would
        --25                         Parse as Ruby 2.5 would
        --26                         Parse as Ruby 2.6 would
        --27                         Parse as Ruby 2.7 would
        --28                         Parse as Ruby 2.8 would
        --mac                        Parse as MacRuby 0.12 would
        --ios                        Parse as mid-2015 RubyMotion would
        --legacy                     Parse with all legacy modes
        --legacy-lambda              Parse with legacy mode for emit_lambda
        --legacy-procarg0            Parse with legacy mode for emit_procarg0
        --legacy-encoding            Parse with legacy mode for emit_encoding
        --legacy-index               Parse with legacy mode for emit_index
        --legacy-arg-inside-procarg0 Parse with legacy mode for emit_arg_inside_procarg0
    -w, --warnings                   Enable warnings
    -B, --benchmark                  Benchmark the processor
    -e fragment                      Process a fragment of Ruby code
    -L, --locate                     Explain how source maps for AST nodes are laid out
    -E, --explain                    Explain how the source is tokenized
        --emit-ruby                  Emit S-expressions as valid Ruby code
        --emit-json                  Emit S-expressions as valid JSON
    -h, --help                       Display this help message and exit
    -V, --version                    Output version information and exit

  If you specify a DIRECTORY, then all *.rb files are fetched
  from it recursively and appended to the file list.

  The default parsing mode is for current Ruby (2.7.1).

[marcandre]

(If this is accepted, a release would be much appreciated)

[iliabylich]

👍

[iliabylich]

@marcandre 2.7.1.3 has been released - https://rubygems.org/gems/parser/versions/2.7.1.3

[marcandre]

Awesome, thank you!