Offset ranges before applying embedded tree-sitter parser

Offset ranges before applying embedded tree-sitter parser

This feature would allow treesitter major modes to easily specify
offsets when using embeded parsers.  A potential use case for this is
javascript template strings, when we want to apply a different parser
to the string's contents, but do not want to include the template
string's delimiters.

* lisp/treesit.el
(treesit-query-range): Accept an optional offest arg, apply the offset
to all returned ranges.
(treesit-range-rules): Accept an optional :offset keyword arg to adjust
ranges an embded parser is applied to.
(treesit-update-ranges): Forward optional :offset setting from
`treesit-range-rules' to `treesit-query-rang'.
* test/lisp/treesit-tests.el
(treesit-range-offset): Tests the new offset functionality.