Add treesit-aggregated-simple-imenu-settings

Add treesit-aggregated-simple-imenu-settings

Now we support setting up Imenu for multiple languages

* doc/lispref/modes.texi: Update manual.
* lisp/treesit.el:
(treesit-aggregated-simple-imenu-settings): New variable.
(treesit--imenu-merge-entries): New function.
(treesit--generate-simple-imenu): This was previously
treesit-simple-imenu.
(treesit-simple-imenu): Support
treesit-aggregated-simple-imenu-settings.
(treesit-major-mode-setup): Recognize
treesit-aggregated-simple-imenu-settings.
* test/src/treesit-tests.el (treesit-imenu): New test.

(cherry picked from commit e2a9af431191d5c71e2ca7a4347ce9e435e8cca0)