[js-api-parser] switch to use `ReviewToken.HasPrefixSpace`

There is a lot of code added to handle whitespace before a token because HasPrefixSpace was not available. This tracks simplifying the js api parser code to use HasPrefixSpace instead.

Start by locating the js-api-parser code that handles whitespace before tokens and inspect how ReviewToken.HasPrefixSpace is exposed and used. Replace the parser's custom whitespace handling with that property, then verify that token parsing behavior remains unchanged across the parser's existing checks.

Written by the indexing model from the issue text.