This keyword will be useful for implementing
string-interpolation feature.
* doc/lispref/modes.texi (Parser-based Font Lock): Document the
new keyword.
* lisp/treesit.el (treesit-font-lock-settings): Document.
(treesit-font-lock-setting-reversed): New function.
(treesit-font-lock-rules): Add new keyword.
(treesit-font-lock-recompute-features): Handle new keyword.
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