Work String#starts_with? with regex (#5485)

* Work String#starts_with? with regex

This feature is imported from Ruby 2.5.0.

See: https://bugs.ruby-lang.org/issues/13712

* Fix String#ends_with to accept regex

See: #5485 (comment)

spec/std/string_spec.cr

@@ -945,6 +945,8 @@ describe "String" do
     it { "foobar".starts_with?("").should be_true }
     it { "foobar".starts_with?("foobarbaz").should be_false }
     it { "foobar".starts_with?("foox").should be_false }
+    it { "foobar".starts_with?(/foo/).should be_true }
+    it { "foobar".starts_with?(/bar/).should be_false }
     it { "foobar".starts_with?('f').should be_true }
     it { "foobar".starts_with?('g').should be_false }
     it { "よし".starts_with?('よ').should be_true }
@@ -956,6 +958,8 @@ describe "String" do
     it { "foobar".ends_with?("").should be_true }
     it { "foobar".ends_with?("foobarbaz").should be_false }
     it { "foobar".ends_with?("xbar").should be_false }
+    it { "foobar".ends_with?(/bar/).should be_true }
+    it { "foobar".ends_with?(/foo|baz/).should be_false }
     it { "foobar".ends_with?('r').should be_true }
     it { "foobar".ends_with?('x').should be_false }
     it { "よし".ends_with?('し').should be_true }

src/string.cr

@@ -4020,6 +4020,10 @@ class String
     false
   end
 
+  def starts_with?(re : Regex)
+    !!($~ = re.match_at_byte_index(self, 0, Regex::Options::ANCHORED))
+  end
+
   def ends_with?(str : String)
     return false if str.bytesize > bytesize
     (to_unsafe + bytesize - str.bytesize).memcmp(str.to_unsafe, str.bytesize) == 0
@@ -4042,6 +4046,10 @@ class String
     true
   end
 
+  def ends_with?(re : Regex)
+    !!($~ = /#{re}\z/.match(self))
+  end
+
   # Interpolates *other* into the string using `Kernel#sprintf`.
   #
   # ```