examples: show how to call a simple v module from ruby (#19073) (#19073)

This is a copy/adaptation of the python example (#13105)

examples/call_v_from_ruby/README.md

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+A simple example to show how to call a function written in v from ruby
+
+Step 1: Compile the v code to a shared library using `v -d no_backtrace -shared test.v`
+
+Step 2: Run the ruby file using `ruby test.rb`
+
+Note: you do not need `-d no_backtrace` if you use gcc or clang .

examples/call_v_from_ruby/test.rb

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+require 'bundler/inline'
+
+gemfile do
+  source 'https://rubygems.org'
+  gem 'ffi'
+end
+
+require 'ffi'
+
+module Lib
+  extend FFI::Library
+
+  ffi_lib File.join(File.dirname(__FILE__), 'test.so')
+
+  attach_function :square, [:int], :int
+  attach_function :sqrt_of_sum_of_squares, [:double, :double], :double
+end
+
+puts "Lib.square(10) result is #{Lib.square(10)}"
+raise 'Cannot validate V square().' unless Lib.square(10) == 100
+
+raise 'Cannot validate V sqrt_of_sum_of_squares().' unless \
+  Lib.sqrt_of_sum_of_squares(1.1, 2.2) == Math.sqrt(1.1*1.1 + 2.2*2.2)

examples/call_v_from_ruby/test.v

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+module test
+
+import math
+
+[export: 'square']
+fn square(i int) int {
+	return i * i
+}
+
+[export: 'sqrt_of_sum_of_squares']
+fn sqrt_of_sum_of_squares(x f64, y f64) f64 {
+	return math.sqrt(x * x + y * y)
+}