Make Monitor a core class

[Feature #21788]

It allows monitor to access internal routines and remove some overhead.

Before:

```
ruby 4.0.0dev (2025-12-13T04:52:13Z master 71dd272506) +YJIT +PRISM [arm64-darwin25]
Warming up --------------------------------------
               Mutex     2.111M i/100ms
             Monitor     1.736M i/100ms
Calculating -------------------------------------
               Mutex     25.050M (± 0.4%) i/s   (39.92 ns/i) -    126.631M in   5.055208s
             Monitor     19.809M (± 0.1%) i/s   (50.48 ns/i) -    100.672M in   5.082015s
```

After:

```
ruby 4.0.0dev (2025-12-13T06:49:18Z core-monitor 6fabf389fd) +YJIT +PRISM [arm64-darwin25]
Warming up --------------------------------------
               Mutex     2.144M i/100ms
             Monitor     1.859M i/100ms
Calculating -------------------------------------
               Mutex     24.771M (± 0.4%) i/s   (40.37 ns/i) -    124.342M in   5.019716s
             Monitor     23.722M (± 0.4%) i/s   (42.15 ns/i) -    118.998M in   5.016361s
```

Bench:

```ruby
require 'bundler/inline'

gemfile do
  gem "benchmark-ips"
end

mutex = Mutex.new
require "monitor"
monitor = Monitor.new

Benchmark.ips do |x|
  x.report("Mutex") { mutex.synchronize { } }
  x.report("Monitor") { monitor.synchronize { } }
end
```

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