[lexical] Chore: Added missing isInline function to TextNode

[mrdivyansh]

Description

LexicalNode expects the extender Node to implement isInline. However, it is missing from TextNode implementation.

lexical/packages/lexical/src/LexicalNode.ts

Lines 317 to 323 in 054fb46

	isInline(): boolean {
	invariant(
	false,
	'LexicalNode: Node %s does not implement .isInline().',
	this.constructor.name,
	);
	}

Test plan

Before

• Invoke isInline function on a TextNode instance.
• Invariant will throw an error -- LexicalNode: Node TextNode does not implement .isInline().

After

• Invoke isInline function on a TextNode instance.
• The function should return true.

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[mrdivyansh]

Not sure if this PR requires any unit tests.

[etrepum]

This might make sense, but there should already be guards to prevent this from happening on nodes other than ElementNode and DecoratorNode. Did you run into a situation where this exception occurs? If so, that’s where the unit test should be. I think otherwise it might make more sense to move this to the ElementNode and DecoratorNode interfaces specifically because those are the only two types of nodes where it may return something meaningful.

[etrepum]

If this does make sense to implement on TextNode, it would make sense to also narrow the implementation to return only true, because it's not permitted for any subclass to return false

Suggested change

	isInline(): boolean {
	isInline(): true {

[etrepum]

Also if we do decide to implement this, there's a fourth node that extends LexicalNode that should implement isInline: LineBreakNode. That would cover all of the expected cases, since the value for LexicalNode isn't exported there's no reasonable way to add another class to the hierarchy.