Initial support for Angular expression syntax

This commit...

1. adds syntax definitions for Angular expression syntax,
   which is a subset of JavaScript.
   see: https://angular.dev/guide/templates/expression-syntax

   Required rules are directly implemented in favor of re-using/including
   JavaScript to reduce dependencies and keep syntax lean (from parser's
   point of view).

2. adds syntax based folding rules to be able to fold control flow blocks
   regardless used indentation style.

3. adds symbol list definition to index `@let` variables.

Fold.tmPreferences

@@ -0,0 +1,51 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<plist version="1.0">
+<dict>
+    <key>scope</key>
+    <string>text.html.ngx</string>
+    <key>settings</key>
+    <dict>
+        <key>foldScopes</key>
+        <array>
+            <dict>
+                <key>begin</key>
+                <string>punctuation.definition.comment.begin</string>
+                <key>end</key>
+                <string>punctuation.definition.comment.end</string>
+                <key>excludeTrailingNewlines</key>
+                <false/>
+            </dict>
+            <dict>
+                <key>begin</key>
+                <string>punctuation.section.block.begin</string>
+                <key>end</key>
+                <string>punctuation.section.block.end</string>
+            </dict>
+            <dict>
+                <key>begin</key>
+                <string>punctuation.section.embedded.begin</string>
+                <key>end</key>
+                <string>punctuation.section.embedded.end</string>
+            </dict>
+            <dict>
+                <key>begin</key>
+                <string>punctuation.section.group.begin</string>
+                <key>end</key>
+                <string>punctuation.section.group.end</string>
+            </dict>
+            <dict>
+                <key>begin</key>
+                <string>punctuation.section.mapping.begin</string>
+                <key>end</key>
+                <string>punctuation.section.mapping.end</string>
+            </dict>
+            <dict>
+                <key>begin</key>
+                <string>punctuation.section.sequence.begin</string>
+                <key>end</key>
+                <string>punctuation.section.sequence.end</string>
+            </dict>
+        </array>
+    </dict>
+</dict>
+</plist>