Implement Hash.new(capacity:)

[Feature #19236]

When building a large hash, pre-allocating it with enough
capacity can save many re-hashes and significantly improve
performance.

```
/opt/rubies/3.3.0/bin/ruby --disable=gems -rrubygems -I./benchmark/lib ./benchmark/benchmark-driver/exe/benchmark-driver \
	            --executables="compare-ruby::../miniruby-master -I.ext/common --disable-gem" \
	            --executables="built-ruby::./miniruby --disable-gem" \
	            --output=markdown --output-compare -v $(find ./benchmark -maxdepth 1 -name 'hash_new' -o -name '*hash_new*.yml' -o -name '*hash_new*.rb' | sort)
compare-ruby: ruby 3.4.0dev (2024-03-25T11:48:11Z master f53209f023) +YJIT dev [arm64-darwin23]
last_commit=[ruby/irb] Cache RDoc::RI::Driver.new (ruby/irb#911)
built-ruby: ruby 3.4.0dev (2024-03-25T15:29:40Z hash-new-rb 77652b08a2) +YJIT dev [arm64-darwin23]
warming up...

|                    |compare-ruby|built-ruby|
|:-------------------|-----------:|---------:|
|new                 |      7.614M|    5.976M|
|                    |       1.27x|         -|
|new_with_capa_1k    |     13.931k|   15.698k|
|                    |           -|     1.13x|
|new_with_capa_100k  |     124.746|   148.283|
|                    |           -|     1.19x|
```

core/hash/new_spec.rb

@@ -34,7 +34,7 @@
     -> { Hash.new(nil) { 0 } }.should raise_error(ArgumentError)
   end
 
-  ruby_version_is "3.3" do
+  ruby_version_is "3.3"..."3.4" do
     it "emits a deprecation warning if keyword arguments are passed" do
       -> { Hash.new(unknown: true) }.should complain(
         Regexp.new(Regexp.escape("Calling Hash.new with keyword arguments is deprecated and will be removed in Ruby 3.4; use Hash.new({ key: value }) instead"))
@@ -46,4 +46,22 @@
       Hash.new({ unknown: true }).default.should == { unknown: true }
     end
   end
+
+  ruby_version_is "3.4" do
+    it "accepts a capacity: argument" do
+      Hash.new(5, capacity: 42).default.should == 5
+      Hash.new(capacity: 42).default.should == nil
+      (Hash.new(capacity: 42) { 1 }).default_proc.should_not == nil
+    end
+
+    it "ignores negative capacity" do
+      -> { Hash.new(capacity: -42) }.should_not raise_error
+    end
+
+    it "raises an error if unknown keyword arguments are passed" do
+      -> { Hash.new(unknown: true) }.should raise_error(ArgumentError)
+      -> { Hash.new(1, unknown: true) }.should raise_error(ArgumentError)
+      -> { Hash.new(unknown: true) { 0 } }.should raise_error(ArgumentError)
+    end
+  end
 end