feat(treesitter): add next, prev sibling method

Add tsnode methods to change to the next, previous, named or unnamed
nodes.

test/functional/treesitter/node_spec.lua

@@ -0,0 +1,62 @@
+local helpers = require('test.functional.helpers')(after_each)
+
+local clear = helpers.clear
+local eq = helpers.eq
+local exec_lua = helpers.exec_lua
+local insert = helpers.insert
+local pending_c_parser = helpers.pending_c_parser
+
+before_each(clear)
+
+local function lua_eval(lua_expr)
+  return exec_lua("return " .. lua_expr)
+end
+
+describe('treesitter node API', function()
+  clear()
+
+  if pending_c_parser(pending) then
+    return
+  end
+
+  it('can move between siblings', function()
+    insert([[
+      int main(int x, int y, int z) {
+        return x + y * z
+      }
+    ]])
+
+    exec_lua([[
+      query = require"vim.treesitter.query"
+      parser = vim.treesitter.get_parser(0, "c")
+      tree = parser:parse()[1]
+      root = tree:root()
+      lang = vim.treesitter.inspect_language('c')
+
+      function node_text(node)
+        return query.get_node_text(node, 0)
+      end
+    ]])
+
+    exec_lua 'node = root:descendant_for_range(0, 11, 0, 16)'
+    eq('int x', lua_eval('node_text(node)'))
+
+    exec_lua 'node = node:next_sibling()'
+    eq(',', lua_eval('node_text(node)'))
+
+    exec_lua 'node = node:next_sibling()'
+    eq('int y', lua_eval('node_text(node)'))
+
+    exec_lua 'node = node:prev_sibling()'
+    eq(',', lua_eval('node_text(node)'))
+
+    exec_lua 'node = node:prev_sibling()'
+    eq('int x', lua_eval('node_text(node)'))
+
+    exec_lua 'node = node:next_named_sibling()'
+    eq('int y', lua_eval('node_text(node)'))
+
+    exec_lua 'node = node:prev_named_sibling()'
+    eq('int x', lua_eval('node_text(node)'))
+  end)
+end)