Optimize Base.callbacks_needed

This matters since there are quite a few public methods per cop class.
In total, it gets called ~57k times, or ~104 times per cop class.

`method_defined?` is faster than string checking, so move that first. It also eliminates most of the checks, Base contains 98 methods.

Current:
```
$ hyperfine -w 10 -r 25 "bundle exec rubocop Rakefile --cache=false"
Benchmark 1: bundle exec rubocop Rakefile --cache=false
  Time (mean ± σ):     932.8 ms ±   7.6 ms    [User: 828.0 ms, System: 104.0 ms]
  Range (min … max):   919.5 ms … 952.2 ms    25 runs
```

With String:
```
$ hyperfine -w 10 -r 25 "bundle exec rubocop Rakefile --cache=false"
Benchmark 1: bundle exec rubocop Rakefile --cache=false
  Time (mean ± σ):     924.1 ms ±   7.8 ms    [User: 816.3 ms, System: 107.3 ms]
  Range (min … max):   912.8 ms … 941.5 ms    25 runs
```

With String + order switched:
```
$ hyperfine -w 10 -r 25 "bundle exec rubocop Rakefile --cache=false"
Benchmark 1: bundle exec rubocop Rakefile --cache=false
  Time (mean ± σ):     920.7 ms ±   6.9 ms    [User: 814.5 ms, System: 105.6 ms]
  Range (min … max):   906.9 ms … 932.5 ms    25 runs
```

~ 1.3% faster

lib/rubocop/cop/base.rb

@@ -322,8 +322,12 @@ def callbacks_needed
       # @api private
       def self.callbacks_needed
         @callbacks_needed ||= public_instance_methods.select do |m|
-          m.start_with?(/on_|after_/) &&
-            !Base.method_defined?(m) # exclude standard "callbacks" like 'on_begin_investigation'
+          # OPTIMIZE: Check method existence first to make fewer `start_with?` calls.
+          # At the time of writing this comment, this excludes 98 of ~104 methods.
+          # `start_with?` with two string arguments instead of a regex is faster
+          # in this specific case as well.
+          !Base.method_defined?(m) && # exclude standard "callbacks" like 'on_begin_investigation'
+            m.start_with?('on_', 'after_')
         end
       end
       # rubocop:enable Layout/ClassStructure