patch 9.1.1169: using global variable for get_insert()/get_lambda_name()

patch 9.1.1169: using global variable for get_insert()/get_lambda_name()

Problem:  using global variable for get_insert()/get_lambda_name()
          (after v9.1.1151)
Solution: let the functions return a string_T object instead
          (Yee Cheng Chin)

In #16720, `get_insert()` was modified to store a string length in a
global variable to be queried immediately by another `get_insert_len()`
function, which is somewhat fragile. Instead, just have the function
itself return a `string_T` object instead. Also do the same for
`get_lambda_name()` which has similar issues.

closes: #16775

Signed-off-by: Yee Cheng Chin <ychin.git@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>