Enhancement: finer control of word separators

<!-- Please search existing issues to avoid creating duplicates. --> <!-- Describe the feature you'd like. --> Currently, the setting term `"editor.wordSeparators"` only tells the editor to separate words *at* these characters. Can we make it *before* or *after* the separators? Now I take LaTeX as an example. If we remove `\` from "editor.wordSeparators", then double click on `\cmdA\cmdB\cmdC`, we will select the whole expression. However, as the expression is actually three commands, we should select a single (e.g. `\cmdA`) one at a time. That means, we should only separate words *before* `\`, not after.