feat(analysis/asymptotics): define `is_Theta` (#14567)
* define `f =Θ[l] g` and prove basic properties;
* add `is_O.const_smul_left`, `is_o.const_smul_left`;
* rename `is_O_const_smul_left_iff` and `is_o_const_smul_left_iff` to
`is_O_const_smul_left` and `is_o_const_smul_left`.
