Provide a way to specify existing bindings when analyzing an interactive document

[alexrp]

Also, we need to process let statements in a similar fashion to what we do in block expressions:

celerity/src/language/core/Semantics/LanguageAnalyzer.cs

Lines 279 to 284 in 8172942

	public override InteractiveDocumentSemantics VisitInteractiveDocument(InteractiveDocumentSyntax node)
	{
	var subs = ConvertList(node.Submissions, static (@this, sub) => @this.VisitSubmission(sub));
	return new(node, subs);
	}

celerity/src/language/core/Semantics/LanguageAnalyzer.cs

Lines 854 to 884 in 8172942

	public override BlockExpressionSemantics VisitBlockExpression(BlockExpressionSyntax node)
	{
	using var ctx = PushScope<BlockScope>();
	var stmts = Builder<StatementSemantics>(node.Statements.Count);
	// Let statements are somewhat special in that they introduce a 'horizontal' scope in the tree; that is,
	// bindings in a let statement become available to siblings to the right of the let statement.
	var lets = new List<ScopeContext<Scope>>();
	var defers = ctx.Scope.DeferStatements;
	foreach (var stmt in node.Statements)
	{
	if (stmt is LetStatementSyntax)
	lets.Add(PushScope<Scope>());
	var sema = VisitStatement(stmt);
	if (sema is DeferStatementSemantics defer)
	defers.Add(defer);
	stmts.Add(sema);
	}
	for (var i = lets.Count - 1; i >= 0; i--)
	lets[i].Dispose();
	defers.Reverse();
	return new(node, List(node.Statements, stmts), defers.DrainToImmutable());
	}