Update the Enso grammar in the VS Code extension (#7625)

Following up on #7539 which introduced great improvements to the Enso grammar, I add some more to try to make it even better.

- Changing to highlight `self` instead of `this` (as the keyword was renamed a long time ago).
- Trying to include the string quotes to be highlighted as part of the string - a bit more controversial but IMO it looks more consistent with simple literals.

Before:
![image](https://github.com/enso-org/enso/assets/1436948/7d455c22-042a-4b43-a603-d661190aa7a4)

After:
![image](https://github.com/enso-org/enso/assets/1436948/5bfab6c0-ad67-4302-b5d3-0ef2fce232a9)

tools/enso4igv/src/main/resources/org/enso/tools/enso4igv/enso.tmLanguage.json

@@ -21,7 +21,7 @@
 		"keywords": {
 			"patterns": [{
 					"name": "keyword.other",
-					"match": "\\b(here|this|type)\\b"
+					"match": "\\b(self|type)\\b"
 				}, {
 					"name": "keyword.control",
 					"match": "\\b(case|of|if|then|else)\\b"
@@ -88,12 +88,22 @@
 				{
 					"contentName": "string.quoted.triple.begin",
 					"begin": "^(\\s*)(\"\"\"|''')",
-					"end": "^(?!\\1\\s+)(?!\\s*$)"
+					"end": "^(?!\\1\\s+)(?!\\s*$)",
+					"beginCaptures": {
+						"2": {
+							"name": "string.quoted.triple.begin"
+						}
+					}
 				},
 				{
 					"contentName": "string.quoted.triple.middle",
 					"begin": "^(\\s*)[^\\s].*(\"\"\"|''')",
-					"end": "^(?!\\1\\s+)(?!\\s*$)"
+					"end": "^(?!\\1\\s+)(?!\\s*$)",
+					"beginCaptures": {
+						"2": {
+							"name": "string.quoted.triple.begin"
+						}
+					}
 				},
 				{
 					"name": "string.quoted.single",