[Parser] Simplify sequence expression parsing

Using `UnresolvedTernaryExpr`/`UnresolvedAsExpr`/`UnresolvedIsExpr`,
`SequenceExprSytnax` now gurantees that it has odd number of elments.

Factored out the operator parsing to `parseSequenceExpressionOperator()`
which returns the operator expresssion, and optionally the RHS.

Author: rintaro