Make LuaTable.Clear method that preserves LuaTable capacity

[JagYayu]

I hope that the LuaTable.Clear function can preserve the LuaTable’s original capacity after clearing all elements, without releasing the internal heap data -- similar to table.clear in LuaJIT.
This would allow me to reuse tables in high-frequency scenarios and reduce GC pressure by using cached/recycled tables.

[neolithos]

This might be a slow version of clear, because all entries must be set to null.

I but this issue on my list.