Support for @-syntax for attributes (#500)

Before:

![image](https://github.com/user-attachments/assets/2b54ad18-3bd8-45b0-874b-2bad954706af)

After:

![image](https://github.com/user-attachments/assets/5d4fe1c9-42df-4c55-8680-3e4a66749595)

Related issue
dafny-lang/dafny#5795

@-attributes are now ready on Dafny, and they also have auto-complete
from the language server. Having syntax hightlight will make them shine.
I added a "visual regression test". It's unfortunate that they are not
programmatically tested, but I hope that we will have semantic
highlighting that will get rid of that issue.

Author: MikaelMayer

src/test/highlight/regression-highlight.dfy

@@ -1,3 +1,6 @@
+// This attribute should be colored
+@IsolateAssertions
+
 // Issue #386
 method Test(thisNat: nat, dothis: nat) {
           //^^^^            ^^^^ this should not be highlighted separatedly in blue

syntaxes/Dafny.tmLanguage

@@ -262,6 +262,49 @@
 		<dict>
 			<key>patterns</key>
 			<array>
+			  <dict>
+					<key>begin</key>
+					<string>@([\w'\?]+)\(</string>
+					<key>beginCaptures</key>
+					<dict>
+						<key>1</key>
+						<dict>
+							<key>name</key>
+							<string>entity.other.attribute-name.dafny</string>
+						</dict>
+					</dict>
+					<key>end</key>
+					<string>\)</string>
+					<key>name</key>
+					<string>attribute</string>
+					<key>patterns</key>
+					<array>
+						<dict>
+							<key>include</key>
+							<string>#code</string>
+						</dict>
+					</array>
+				</dict>
+				
+			  <dict>
+					<key>begin</key>
+					<string>@([\w'\?]+)(?!\()</string>
+					<key>beginCaptures</key>
+					<dict>
+						<key>1</key>
+						<dict>
+							<key>name</key>
+							<string>entity.other.attribute-name.dafny</string>
+						</dict>
+					</dict>
+					<key>end</key>
+					<string>(?<=[\w'\?]+)</string>
+					<key>name</key>
+					<string>attribute</string>
+					<key>patterns</key>
+					<array>
+					</array>
+				</dict>
 				<dict>
 					<key>begin</key>
 					<string>\{:(\w+)</string>