add benchmark/time_at.yml

```
                                ruby_2_6    ruby_2_7    ruby_3_0      master    modified
                   Time.at(0)    12.362M     11.015M      9.499M      6.615M      9.000M i/s -     32.115M times in 2.597946s 2.915517s 3.380725s 4.854651s 3.568234s
              Time.at(0, 500)     7.542M      7.136M      8.252M      5.707M      5.646M i/s -     20.713M times in 2.746279s 2.902556s 2.510166s 3.629644s 3.668854s
     Time.at(0, in: "+09:00")     1.426M      1.346M      1.565M      1.674M      1.667M i/s -      4.240M times in 2.974049s 3.149753s 2.709416s 2.533043s 2.542853s
```

```
ruby_2_6: ruby 2.6.7p150 (2020-12-09 revision 67888) [x86_64-linux]
ruby_2_7: ruby 2.7.3p140 (2020-12-09 revision 9b884df6dd) [x86_64-linux]
ruby_3_0: ruby 3.0.3p150 (2021-11-06 revision 6d540c1b98) [x86_64-linux]
master: ruby 3.1.0dev (2021-11-13T20:48:57Z master fc456adc6a) [x86_64-linux]
modified: ruby 3.1.0dev (2021-11-15T01:12:51Z mandatory_only_bui.. b0228446db) [x86_64-linux]
```

benchmark/time_at.yml

@@ -0,0 +1,5 @@
+benchmark:
+  - 'Time.at(0)'
+  - 'Time.at(0, 500)'
+  - 'Time.at(0, in: "+09:00")'
+  - 'Time.at(0, 500, in: "+09:00")'