Add XML.libxml2_version (#15623)

As discussed in #13273, we're missing a consistent format for exposing version information.
Returning the version number formatted as a string seems the most expected variant, though.

We might consider adding extra information as well, but I'm not sure about that.

Author: straight-shoota

spec/std/xml/xml_spec.cr

@@ -609,4 +609,8 @@ describe XML do
       end.should eq %[<foo><bar/></foo>\n]
     end
   end
+
+  it ".libxml2_version" do
+    XML.libxml2_version.should match /2\.\d+\.\d+/
+  end
 end

src/xml.cr

@@ -125,6 +125,17 @@ module XML
       ptr.value = old
     end
   end
+
+  class_getter libxml2_version : String do
+    version_string = String.new(LibXML.xmlParserVersion)
+
+    # The version string can contain extra information after the version number,
+    # so we ignore any trailing non-numbers with `strict: false`
+    number = version_string.to_i(strict: false)
+
+    # Construct a formatted version string
+    "#{number // 10_000}.#{number % 10_000 // 100}.#{number % 100}"
+  end
 end
 
 require "./xml/*"

src/xml/libxml2.cr

@@ -19,6 +19,8 @@ require "./save_options"
 lib LibXML
   alias Int = LibC::Int
 
+  $xmlParserVersion : LibC::Char*
+
   fun xmlInitParser
 
   fun __xmlIndentTreeOutput : Int*