(PowerShellGH-1336) Add syntax folding for here strings

This commit adds detection of text regions composed of single and double
quoted here strings;  @' '@ and @" "@.

Author: glennsarti

src/features/Folding.ts

@@ -182,6 +182,31 @@ export class FoldingProvider implements vscode.FoldingRangeProvider {
         return result.reverse();
     }
 
+    // Given a textmate scope name, find a series of contiguous tokens which contain
+    // that scope name and pair them together.
+    private matchContiguousScopeElements(tokens,
+                                         scopeName: string,
+                                         matchType: MatchType,
+                                         document: vscode.TextDocument) {
+        const result = [];
+        let startToken;
+
+        tokens.forEach((token, index) => {
+            if (token.scopes.includes(scopeName)) {
+                if (startToken === undefined) { startToken = token; }
+
+                // If we're at the end of the token list, or the next token does not include the scopeName
+                // we've reached the end of the contiguous block.
+                if (((index + 1) >= tokens.length) || (!tokens[index + 1].scopes.includes(scopeName))) {
+                    result.push(new MatchedToken(startToken, token, null, null, matchType, document));
+                    startToken = undefined;
+                }
+            }
+        });
+
+        return result;
+    }
+
     // Given a list of tokens, return a list of matched tokens/line numbers regions
     private matchGrammarTokens(tokens, document: vscode.TextDocument): IMatchedTokenList {
         const matchedTokens = [];
@@ -202,6 +227,20 @@ export class FoldingProvider implements vscode.FoldingRangeProvider {
             MatchType.Region, document)
             .forEach((match) => { matchedTokens.push(match); });
 
+        // Find contiguous here strings   @' -> '@
+        this.matchContiguousScopeElements(
+            tokens,
+            "string.quoted.single.heredoc.powershell",
+            MatchType.Region, document)
+            .forEach((match) => { matchedTokens.push(match); });
+
+        // Find contiguous here strings   @" -> "@
+        this.matchContiguousScopeElements(
+            tokens,
+            "string.quoted.double.heredoc.powershell",
+            MatchType.Region, document)
+            .forEach((match) => { matchedTokens.push(match); });
+
         return matchedTokens;
     }