LibSQL: Parse most language expressions

https://sqlite.org/lang_expr.html

The entry point to using expressions, parse_expression(), is not used
by SQL::Parser in this commit. But there's so much here that it's easier
to grok as its own commit.

Userland/Libraries/LibSQL/Forward.h

@@ -28,14 +28,37 @@
 
 namespace SQL {
 class ASTNode;
+class BetweenExpression;
+class BinaryOperatorExpression;
+class BlobLiteral;
+class CaseExpression;
+class CastExpression;
+class ChainedExpression;
+class CollateExpression;
 class ColumnDefinition;
+class ColumnNameExpression;
 class CreateTable;
 class DropTable;
+class ErrorExpression;
 class ErrorStatement;
+class Expression;
+class InChainedExpression;
+class InTableExpression;
+class InvertibleNestedDoubleExpression;
+class InvertibleNestedExpression;
+class IsExpression;
 class Lexer;
+class MatchExpression;
+class NestedDoubleExpression;
+class NestedExpression;
+class NullExpression;
+class NullLiteral;
+class NumericLiteral;
 class Parser;
 class SignedNumber;
 class Statement;
+class StringLiteral;
 class Token;
 class TypeName;
+class UnaryOperatorExpression;
 }