Re-enable must_use clippy rule

[tunz]

Following up to re-enable must_use clippy rule for boa_engine which was disabled in #3109.

I gave an exception to set_prototype because it returns false when it's not extensible but there are some code already checking if it's extensible, so they don't need to check the return value, and it looks like that's a common pattern.

[codecov]

Codecov Report

Merging #3180 (268b175) into main (33e8c51) will not change coverage.
Report is 3 commits behind head on main.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #3180   +/-   ##
=======================================
  Coverage   50.50%   50.50%           
=======================================
  Files         443      443           
  Lines       42712    42712           
=======================================
  Hits        21572    21572           
  Misses      21140    21140           

Files Changed	Coverage Δ
boa_engine/src/builtins/function/mod.rs	39.88% <ø> (ø)
boa_engine/src/builtins/iterable/mod.rs	71.42% <ø> (ø)
boa_engine/src/builtins/map/ordered_map.rs	76.92% <ø> (ø)
boa_engine/src/builtins/promise/mod.rs	30.07% <ø> (ø)
boa_engine/src/builtins/set/ordered_set.rs	69.09% <ø> (ø)
...src/builtins/typed_array/integer_indexed_object.rs	0.00% <ø> (ø)
boa_engine/src/context/intrinsics.rs	98.09% <ø> (ø)
boa_engine/src/context/mod.rs	54.98% <ø> (ø)
boa_engine/src/error.rs	33.48% <ø> (ø)
boa_engine/src/job.rs	47.05% <ø> (ø)
... and 16 more

[HalidOdat]

Looks perfect to me :)