[C#] Add 'required' keyword

Resolves #3926

This commit adds `required` modifier which has been added by C#11.

see: https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/required

C#/C#.sublime-syntax

@@ -40,7 +40,7 @@ variables:
   brackets_capture: '((\[)(,*)(\]))'
   type_suffix_capture: '(\?)?{{brackets_capture}}?(?:\s*(\*))?'
 
-  reserved: '(?:abstract|as|base|break|case|catch|checked|class|const|continue|default|delegate|do|else|enum|event|explicit|extern|finally|fixed|for|foreach|goto|if|implicit|in|interface|internal|is|lock|nameof|namespace|new|not|null|operator|out|override|params|private|protected|public|readonly|ref|return|sealed|sizeof|stackalloc|static|string|struct|switch|this|throw|try|typeof|unchecked|unsafe|using|virtual|volatile|while)'
+  reserved: '(?:abstract|as|base|break|case|catch|checked|class|const|continue|default|delegate|do|else|enum|event|explicit|extern|finally|fixed|for|foreach|goto|if|implicit|in|interface|internal|is|lock|nameof|namespace|new|not|null|operator|out|override|params|private|protected|public|readonly|ref|required|return|sealed|sizeof|stackalloc|static|string|struct|switch|this|throw|try|typeof|unchecked|unsafe|using|virtual|volatile|while)'
   name: '(?:@{{reserved}}|@{{base_type}}|@var|@?{{name_normal}})'
   namespaced_name: (?:(?:{{name}}{{generic_declaration}}\s*\.\s*)*{{name}}{{generic_declaration}})
 
@@ -225,7 +225,7 @@ contexts:
     # allows coloration of code outside a class
     - match: (?=\S)
       push:
-        - match: (?={{visibility}}|\b(?:class|delegate|interface|namespace|readonly|record|static)\b)
+        - match: (?={{visibility}}|\b(?:class|delegate|interface|namespace|readonly|record|required|static)\b)
           pop: true
         - include: line_of_code
 
@@ -334,7 +334,7 @@ contexts:
     - include: main
 
   class_declaration:
-    - match: '\b(static|unsafe|abstract|partial|sealed)\b'
+    - match: \b(static|unsafe|abstract|partial|required|sealed)\b
       scope: storage.modifier.cs
     - match: '{{visibility}}'
       scope: storage.modifier.access.cs

C#/tests/syntax_test_C#11.cs

@@ -134,3 +134,25 @@ The following example shows how newlines can improve the readability of an expre
 ///         ^^^^^^ punctuation.definition.string.begin
 ///                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ - constant
 ///                                                            ^^^ punctuation.definition.string.end
+
+public class C2 {
+/// <- storage.modifier.access
+///    ^^^^^ keyword.declaration.class
+///          ^^ entity.name.class
+
+    public required string FirstName { get; init; }
+/// ^^^^^^ storage.modifier.access
+///        ^^^^^^^^ storage.modifier
+///                 ^^^^^^ storage.type
+}
+
+public struct S2 {
+/// <- storage.modifier.access
+///    ^^^^^^ keyword.declaration.struct
+///           ^^ entity.name.struct
+
+    public required string FirstName { get; init; }
+/// ^^^^^^ storage.modifier.access
+///        ^^^^^^^^ storage.modifier
+///                 ^^^^^^ storage.type
+}

C#/tests/syntax_test_C#7.cs

@@ -7,6 +7,12 @@ class Foo {
 ///^^^^^^^^ meta.class
 ///       ^ meta.class.body
 
+    public readonly double value;
+/// ^^^^^^ storage.modifier.access
+///        ^^^^^^^^ storage.modifier
+///                 ^^^^^^ storage.type
+///                        ^^^^^ variable.other.member
+
     void Main(string[] args) {
 /// ^^^^ storage.type
 ///      ^^^^^^^^^^^^^^^^^^^^^ meta.method