Align naming of FindTreeSitter cmake file and variables

`find_package(Foo ...)` expects to find a file FindFoo.cmake and the
resulting variables to be named `Foo_...`.  If those don't all match up,
then the detection does not work properly.

Closes neovim#13262

Author: jamessan

CMakeLists.txt

@@ -374,8 +374,8 @@ include_directories(SYSTEM ${MSGPACK_INCLUDE_DIRS})
 find_package(LibLUV 1.30.0 REQUIRED)
 include_directories(SYSTEM ${LIBLUV_INCLUDE_DIRS})
 
-find_package(Treesitter REQUIRED)
-include_directories(SYSTEM ${TREESITTER_INCLUDE_DIRS})
+find_package(TreeSitter REQUIRED)
+include_directories(SYSTEM ${TreeSitter_INCLUDE_DIRS})
 
 # Note: The test lib requires LuaJIT; it will be skipped if LuaJIT is missing.
 option(PREFER_LUA "Prefer Lua over LuaJIT in the nvim executable." OFF)

cmake/FindTreeSitter.cmake

@@ -0,0 +1,11 @@
+# - Try to find tree-sitter
+# Once done, this will define
+#
+#  TreeSitter_FOUND        - system has tree-sitter
+#  TreeSitter_INCLUDE_DIRS - the tree-sitter include directories
+#  TreeSitter_LIBRARIES    - link these to use tree-sitter
+
+include(LibFindMacros)
+
+libfind_pkg_detect(TreeSitter tree-sitter FIND_PATH tree_sitter/api.h FIND_LIBRARY tree-sitter)
+libfind_process(TreeSitter)

cmake/FindTreesitter.cmake

@@ -445,7 +445,7 @@ list(APPEND NVIM_LINK_LIBRARIES
   ${LIBTERMKEY_LIBRARIES}
   ${UNIBILIUM_LIBRARIES}
   ${UTF8PROC_LIBRARIES}
-  ${TREESITTER_LIBRARIES}
+  ${TreeSitter_LIBRARIES}
   ${CMAKE_THREAD_LIBS_INIT}
 )