Move bigdecimal from rbs/stdlib

See also:
- ruby/rbs#2258
- ruby/rbs#817
- ruby/rbs#936
- ruby/rbs#1053

Thank you:
- @ybiquitous
- @takahashim
- @piotaixr

gems/bigdecimal/.rubocop.yml

@@ -0,0 +1,19 @@
+# This configuration inherits from /.rubocop.yml.
+# You can configure RBS style of this gem.
+# This file is used on CI. It is configured to automatically
+# make rubocop suggestions on pull requests for this gem.
+# If you do not like the style enforcement, you should remove this file.
+inherit_from: ../../.rubocop.yml
+
+##
+# If you want to customize the style, please consult with the gem reviewers.
+# You can see the list of cops at https://github.com/ksss/rubocop-on-rbs/blob/main/docs/modules/ROOT/pages/cops.adoc
+
+RBS/Layout:
+  Enabled: true
+
+RBS/Lint:
+  Enabled: true
+
+RBS/Style:
+  Enabled: true

gems/bigdecimal/3.1/_test/test.rb

@@ -0,0 +1,22 @@
+# Write Ruby code to test the RBS.
+# It is type checked by `steep check` command.
+
+require "bigdecimal"
+require "bigdecimal/util"
+
+BigDecimal("1.23")
+
+BigDecimal("1.23") + BigDecimal("1.23")
+BigDecimal("1.23") - BigDecimal("1.23")
+BigDecimal("1.23") * BigDecimal("1.23")
+BigDecimal("1.23") / BigDecimal("1.23")
+
+BigDecimal("1.23").to_d
+BigDecimal("1.23").to_f
+BigDecimal("1.23").to_i
+BigDecimal("1.23").to_r
+
+123.to_d
+12.3.to_d
+12r.to_d(3)
+0i.to_d