fix(parser): support optional start and end indices in slice ranges

Signed-off-by: erick-alcachofa <erick@artichoke.dev>

Update the SliceAccess postfix operator logic to handle the full variety
of slice range syntaxes. This allows for open-ended slices by making the
start and end expressions optional within the brackets.

- Add logic to detect a leading colon for `[:end]` and `[:]` forms.
- Support trailing colons for `[start:]` forms.
- Differentiate between a single index access and a slice range based on
  the presence of the colon operator.
- Update SliceRangeExprNode construction to handle optional boundaries.

lib/src/Parser/Expressions.cpp

@@ -632,6 +632,28 @@ namespace arti::lang {
       node = std::move(newNode);
     }
     else if (op == ast::PostfixOperator::SliceAccess) {
+      bool isSlice = false;
+      bool skipSliceEnd = false;
+      bool skipSliceStart = false;
+
+      if (auto skipLeft = matchAndConsume(TokenV::opColon); ! skipLeft) {
+        return Unexpected<>{ std::move(skipLeft).error() };
+      }
+      else if (skipLeft.value()) {
+        isSlice = true;
+        skipSliceStart = true;
+
+        if (auto close = matchAndConsume(TokenV::opRBracket); ! close) {
+          return Unexpected<>{ std::move(close).error() };
+        }
+        else if (close.value()) {
+          skipSliceEnd = true;
+        }
+      }
+
+      auto idxExpr = ast::Optional<ast::ExpressionNode>{};
+
+      if (! skipSliceStart) {
         auto idx = parseExpression();  
 
         if (! idx) {
@@ -642,6 +664,20 @@ namespace arti::lang {
           return Unexpected<>{ std::move(range).error() };
         }
         else if (range.value()) {
+          isSlice = true;
+
+          if (auto close = matchAndConsume(TokenV::opRBracket); ! close) {
+            return Unexpected<>{ std::move(close).error() };
+          }
+          else if (close.value()) {
+            skipSliceEnd = true;
+          }
+        }
+        
+        idxExpr = std::move(idx).value();
+      }
+
+      if (isSlice) {
         auto newNode = ast::MakeNode<ast::SliceRangeExprNode>();
 
         newNode->location = {
@@ -649,8 +685,11 @@ namespace arti::lang {
           .column = peekToken->column
         };
 
-        newNode->start = std::move(idx).value();
+        if (! skipSliceStart) {
+          newNode->start = std::move(idxExpr).value();
+        }
 
+        if (! skipSliceEnd) {
           auto endIdx = parseExpression();  
 
           if (! endIdx) {
@@ -662,6 +701,7 @@ namespace arti::lang {
           if (auto close = consume(TokenV::opRBracket, "']'"); ! close) {
             return Unexpected<>{ std::move(close).error() };
           }
+        }
 
         newNode->slice = std::move(lhs);
 
@@ -675,7 +715,7 @@ namespace arti::lang {
           .column = peekToken->column
         };
 
-        newNode->index = std::move(idx).value();
+        newNode->index = std::move(idxExpr).value();
 
         if (auto close = consume(TokenV::opRBracket, "']'"); ! close) {
           return Unexpected<>{ std::move(close).error() };