Code highlighting doesn't work in some cases after editing multiline strings

[xonixx]

Discovered after adding fix to #156. Can be relevant to https://intellij-support.jetbrains.com/hc/en-us/community/posts/360010554180--Custom-Language-Plugin-Highlighting-not-working-despite-correctly-parsed-psi-file.

Note, that it parses OK, but highlighting is broken. This happens after removing, then adding back the \.