Replace checking start/end of string

Replace `=~ /^str/`, `=~ /str$/`, `!~ /^str/`, and `!~ /str$/` to `start_with?` and `end_with?` when possible. Mostly it's 1,6x faster, some more impressive benchmarks:

**actionpack/lib/abstract_controller/helpers.rb**:

```ruby
Benchmark.ips do |x|
  x.report('=~') { error.path =~ /^#{path}(\.rb)?$/ }
  x.report('start/end') { error.path.start_with?(path) && error.path.end_with?('.rb', '') }
  x.compare!
end
```

```
Calculating -------------------------------------
                  =~      7097 i/100ms
           start/end     29688 i/100ms
-------------------------------------------------
                  =~   104752.3 (±8.3%) i/s -     525178 in   5.049548s
           start/end  2423935.7 (±10.8%) i/s -   11964264 in   5.003489s

Comparison:
           start/end:  2423935.7 i/s
                  =~:   104752.3 i/s - 23.14x slower
```

--

**actionpack/lib/action_dispatch/journey/visitors.rb**:

```ruby
Benchmark.ips do |x|
  x.report('=~') do
    next unless pim =~ /^visit_(.*)$/
    DISPATCH_CACHE[$1.to_sym] = pim
  end
  x.report('start/end') do
    next unless pim.to_s.start_with?('visit_')
    DISPATCH_CACHE[pim[/^visit_(.*)$/, 1].to_sym] = pim
  end
  x.compare!
end
```

```
Calculating -------------------------------------
                  =~     58993 i/100ms
           start/end     73946 i/100ms
-------------------------------------------------
                  =~  1832025.8 (±6.6%) i/s -    9143915 in   5.012602s
           start/end  2944646.4 (±8.2%) i/s -   14641308 in   5.006457s

Comparison:
           start/end:  2944646.4 i/s
                  =~:  1832025.8 i/s - 1.61x slower
```

--

**actionview/lib/action_view/helpers/asset_tag_helper.rb**:

```ruby
Benchmark.ips do |x|
  x.report('=~') { src =~ /^(?:cid|data):/ }
  x.report('start/end') { src.start_with?('cid:', 'data:') }
  x.compare!
end
```

```
Calculating -------------------------------------
                  =~     26869 i/100ms
           start/end     29965 i/100ms
-------------------------------------------------
                  =~  1853626.9 (±8.5%) i/s -    9189198 in   4.996903s
           start/end  2894114.5 (±13.0%) i/s -   14173445 in   4.993368s

Comparison:
           start/end:  2894114.5 i/s
                  =~:  1853626.9 i/s - 1.56x slower
```

--

**actionview/lib/action_view/renderer/template_renderer.rb**:

```ruby
Benchmark.ips do |x|
  x.report('=~') { layout =~ /^\// }
  x.report('start/end') { layout.start_with?('/') }
  x.compare!
end
```

```
Calculating -------------------------------------
                  =~     25597 i/100ms
           start/end     33362 i/100ms
-------------------------------------------------
                  =~  1451851.0 (±9.4%) i/s -    7192757 in   5.000724s
           start/end  4360476.8 (±14.0%) i/s -   21251594 in   4.996049s

Comparison:
           start/end:  4360476.8 i/s
                  =~:  1451851.0 i/s - 3.00x slower
```

--

**activesupport/lib/active_support/multibyte/chars.rb**:

```ruby
Benchmark.ips do |x|
  x.report('=~') { method.to_s =~ /!$/ }
  x.report('start/end') { method.to_s.end_with?('!') }
  x.compare!
end
```

```
Calculating -------------------------------------
                  =~     25899 i/100ms
           start/end     30125 i/100ms
-------------------------------------------------
                  =~  2046233.9 (±9.2%) i/s -   10126509 in   4.998104s
           start/end  3313337.1 (±12.0%) i/s -   16267500 in   4.994593s

Comparison:
           start/end:  3313337.1 i/s
                  =~:  2046233.9 i/s - 1.62x slower
```
what does the `''` argument accomplish here? what's the meaning of checking if a string "ends with" a blank string?