This commit simplifies the logic of `ExpressionVisitor` in
`completions.ts`.
Specifically,
1. helper functions `uniqueByName` and `lowerName` are removed.
2. Clean up the logic in visitElement()
3. Reorder constructor params
4. Add methods `addAttributeValuesToCompletions`,
`addKeysToCompletions`, and `addSymbolsToCompletions`.
PR Close#33391