fix(tactic/default): import tactic.basic (#3284)

Some basic tactics and commands (e.g. `#explode`) were not available even if `import tactic` was used. I added `import tactic.basic` to `tactic/default.lean` to remedy this.

src/tactic/default.lean

@@ -17,6 +17,7 @@ As (non-exhaustive) examples, these includes things like:
 * data.equiv.encodable
 * order.complete_lattice
 -/
+import tactic.basic -- ensure basic tactics are available
 import tactic.abel
 import tactic.ring_exp
 import tactic.noncomm_ring