cmd/doctor: Add --ignore-warnings

When `brew doctor` is run, `brew` exits with a non-zero status if any
warnings or errors are encountered. However, this groups warnings and
errors together, but (as the output notes) warnings can be innocuous:

> Please note that these warnings are just used to help the Homebrew
> maintainers with debugging if you file an issue. If everything you use
> Homebrew for is working fine: please don't worry or file an issue;
> just ignore this. Thanks!

The `--ignore-warnings` switch makes it possible for automated tools
interacting with Homebrew to distinguish between warnings and hard
errors.

```
$ brew doctor --ignore-warnings
Please note that these warnings are just used to help the Homebrew maintainers
with debugging if you file an issue. If everything you use Homebrew for is
working fine: please don't worry or file an issue; just ignore this. Thanks!

Warning: Some installed casks are deprecated or disabled.
You should find replacements for the following casks:
  graphql-playground
$ echo "$?"
0
```

Hard errors will still cause a non-zero exit status:

```
$ brew doctor --ignore-warnings
Please note that these warnings are just used to help the Homebrew maintainers
with debugging if you file an issue. If everything you use Homebrew for is
working fine: please don't worry or file an issue; just ignore this. Thanks!

Warning: gettext files detected at a system prefix.
These files can cause compilation and link failures, especially if they
are compiled with improper architectures. Consider removing these files:
  /opt/homebrew/lib/libgettextlib.dylib
  /opt/homebrew/lib/libintl.dylib
  /opt/homebrew/include/libintl.h
Error: unknown or unsupported macOS version: :dunno
$ echo "$?"
1
```

Library/Homebrew/cmd/doctor.rb

@@ -23,6 +23,11 @@ class Doctor < AbstractCommand
         switch "-D", "--audit-debug",
                description: "Enable debugging and profiling of audit methods."
 
+        switch "--ignore-warnings",
+               description: "Only exit with a non-zero status if errors are " \
+                            "encountered. Warnings are still displayed but will " \
+                            "not cause the command to fail."
+
         named_args :diagnostic_check
       end
 
@@ -48,7 +53,7 @@ def run
           methods = args.named
         end
 
-        first_warning = T.let(true, T::Boolean)
+        seen_warning = T.let(false, T::Boolean)
         methods.each do |method|
           $stderr.puts Formatter.headline("Checking #{method}", color: :magenta) if args.debug?
           unless checks.respond_to?(method)
@@ -59,7 +64,7 @@ def run
           out = checks.send(method)
           next if out.blank?
 
-          if first_warning
+          unless seen_warning
             $stderr.puts <<~EOS
               #{Tty.bold}Please note that these warnings are just used to help the Homebrew maintainers
               with debugging if you file an issue. If everything you use Homebrew for is
@@ -69,11 +74,11 @@ def run
 
           $stderr.puts
           opoo out
-          Homebrew.failed = true
-          first_warning = false
+          Homebrew.failed = true unless args.ignore_warnings?
+          seen_warning = true
         end
 
-        puts "Your system is ready to brew." if !Homebrew.failed? && !args.quiet?
+        puts "Your system is ready to brew." if !Homebrew.failed? && !seen_warning && !args.quiet?
       end
     end
   end