build: allow to skip treesitter C parser install

if USE_BUNDLED_TS_PARSERS is set to off, don't try to install the
parser.
Distribs can install treesitter parsers directly into $CMAKE_LIBRARY_PATH/nvim
(and users anywhere in rtp).

Also fix the URL.

third-party/cmake/BuildTreesitterParsers.cmake

@@ -1,6 +1,6 @@
 ExternalProject_Add(treesitter-c
 PREFIX ${DEPS_BUILD_DIR}
-URL ${TREESITER_C_URL}
+URL ${TREESITTER_C_URL}
 DOWNLOAD_DIR ${DEPS_DOWNLOAD_DIR}/treesitter-c
 DOWNLOAD_COMMAND ${CMAKE_COMMAND}
   -DPREFIX=${DEPS_BUILD_DIR}