`operator+=` not defined for `var_value<Eigen::VectorXd>`

Description

I got an error with this code:

stan::math::var_value<Eigen::MatrixXd> matrix = Eigen::MatrixXd::Random(5, 5);
stan::math::var_value<Eigen::VectorXd> vector = Eigen::VectorXd::Random(5);

vector = stan::math::add(vector, matrix.col(i));

math/stan/math/rev/core/var.hpp:464:24: warning: inline function 'stan::math::var_value<Eigen::Matrix<double, -1, 1, 0, -1, 1>, void>::operator+=' is not defined [-Wundefined-inline]
  inline var_value<T>& operator+=(const var_value<T>& b);
...
/tmp/vector_read-829dca.o: In function `read_scalar_varmat(benchmark::State&)':
vector_read.cpp:(.text+0xc9a): undefined reference to `stan::math::var_value<Eigen::Matrix<double, -1, 1, 0, -1, 1>, void>::operator+=(stan::math::var_value<Eigen::Matrix<double, -1, 1, 0, -1, 1>, void> const&)'

I ended up using this code:

vector = stan::math::add(vector, matrix.col(i));

But we should just define operator+= (and make sure all the other operators have implementations)

Current Version:

v3.4.0

Start in math/stan/math/rev/core/var.hpp around the declared var_valueEigen::VectorXd::operator+= and inspect the existing var_value operator implementations. Reproduce the linker error from the issue's vector and matrix.col(i) example, then verify that operator+= and the other declared operators have definitions and that the relevant tests pass.