fix(40322): bad completions for string literal type as indexed access type object

[Vboivin]

Fixes #40322

let x: "/*foo*/"["bar"];

Added validation to prevent completion on a string literal when it is the object type.
The cursor at the marker foo will not have autocompletion.

• There is an associated issue in the Backlog milestone (required)
• Code is up-to-date with the master branch
• You've successfully run gulp runtests locally
• There are new or updated unit tests validating the change

[ghost]

All CLA requirements met.

[Vboivin]

@DanielRosenwasser here is the PR for #40322

[DanielRosenwasser]

1. Use spaces after ifs
2. You shouldn't need a type assertion to any anywhere.
3. Just use an early bail-out to return undefined when you're in the wrong node.