feat(syntax): add escaped-quoted-strings token to def syntax

Introduces an escaped-quoted-strings pattern for the def syntax.
It enables proper highlighting of strings with escapes and vars.
The change adds the new pattern to the syntax and fixes the EOF newline.

- Adds escaped-quoted-strings for escapes and embedded vars
- Registers new pattern in repository for proper highlighting
- Ensures trailing newline in the file

syntaxes/def.tmLanguage.json

@@ -5,6 +5,9 @@
 		{
 			"include": "#comment"
 		},
+		{
+			"include": "#escaped-quoted-strings"
+		},
 		{
 			"include": "#strings"
 		},
@@ -28,6 +31,31 @@
 		}
 	],
 	"repository": {
+		"escaped-quoted-strings": {
+			"name": "string.quoted.double.def",
+			"begin": "\\\\\"",
+			"end": "\\\\\"",
+			"beginCaptures": {
+				"0": {
+					"name": "punctuation.definition.string.begin.def"
+				}
+			},
+			"endCaptures": {
+				"0": {
+					"name": "punctuation.definition.string.end.def"
+				}
+			},
+			"patterns": [
+				{
+					"name": "constant.character.escape.def",
+					"match": "\\\\."
+				},
+				{
+					"name": "variable.other.property",
+					"match": "\\$[_a-zA-Z][_a-zA-Z0-9]*(\\(\\$?[_a-zA-Z][_a-zA-Z0-9]*(,\\$?[_a-zA-Z][_a-zA-Z0-9]*)*\\))?"
+				}
+			]
+		},
 		"keywords": {
 			"patterns": [
 				{
@@ -151,4 +179,4 @@
 		}
 	},
 	"scopeName": "source.def"
-}
+}