Don't call `Kernel#require` in hot loop

Ref: https://bugs.ruby-lang.org/issues/20641

Even without the reference bug, `require 'date'` isn't cheap.

```ruby

require "benchmark/ips"
require "yaml"
require "date"

100.times do |i|
  $LOAD_PATH.unshift("/tmp/does/not/exist/#{i}")
end
payload = 100.times.map { Date.today }.to_yaml

Benchmark.ips do |x|
  x.report("100 dates") { YAML.unsafe_load(payload) }
end
```

Before:
```
$ ruby /tmp/bench-yaml.rb
ruby 3.2.2 (2023-03-30 revision e51014f9c0) [arm64-darwin22]
Warming up --------------------------------------
           100 dates   416.000 i/100ms
Calculating -------------------------------------
           100 dates      4.309k (± 1.2%) i/s -     21.632k in   5.021003s
```

After:
```
$ ruby -Ilib /tmp/bench-yaml.rb
ruby 3.2.2 (2023-03-30 revision e51014f9c0) [arm64-darwin22]
Warming up --------------------------------------
           100 dates   601.000 i/100ms
Calculating -------------------------------------
           100 dates      5.993k (± 1.8%) i/s -     30.050k in   5.016079s
```

ext/psych/lib/psych/scalar_scanner.rb

@@ -4,6 +4,8 @@ module Psych
   ###
   # Scan scalars for built in types
   class ScalarScanner
+    autoload :Date, "date"
+
     # Taken from http://yaml.org/type/timestamp.html
     TIME = /^-?\d{4}-\d{1,2}-\d{1,2}(?:[Tt]|\s+)\d{1,2}:\d\d:\d\d(?:\.\d*)?(?:\s*(?:Z|[-+]\d{1,2}:?(?:\d\d)?))?$/
 
@@ -61,7 +63,6 @@ module Psych
           string
         end
       elsif string.match?(/^\d{4}-(?:1[012]|0\d|\d)-(?:[12]\d|3[01]|0\d|\d)$/)
-        require 'date'
         begin
           class_loader.date.strptime(string, '%F', Date::GREGORIAN)
         rescue ArgumentError