Add example to showcase semantic highlighting

Author: xolox

example.lua

@@ -0,0 +1,41 @@
+-- A local variable masking a global one:
+global_variable = '1st value'
+do
+  local global_variable = 'NOT ACTUALLY A GLOBAL!'
+  print(global_variable)
+end
+global_variable = '2nd value'
+
+-- Highlighting for different types of locals:
+local usedlocal = 'this is a local variable'
+local mutated = "this one's assigned multiple times"
+local unused = "and this one isn't referenced anywhere"
+mutated = 'this is the second value of [mutated]'
+print(usedlocal)
+print(undefined)
+
+io.open('')
+string.lower('')
+local tinsert = table.insert
+local test = { field = function(foo, bar, baz) print(baz) end }
+
+-- Highlighting for function arguments:
+local function example(usedparam, mutatedparam, unusedparam)
+  mutatedparam = 42
+  print(usedparam)
+end
+
+-- Argument count checking:
+
+  example(1)
+
+for k, v in pairs(_G) do print(k) end
+
+  example (
+    1,
+    2,
+    3,
+    4)
+
+-- Syntax errors:
+-- example(..)