"hifi sourcemaps" don't work on a new Ember project

On a brand new `5.8.1` project I tried to follow these instructions for enabling hifi sourcemaps: https://nullvoxpopuli.com/2023-12-19-hifi-sourcemap/

This didn't seem to make any change in the sourcemap situation.

In the Ember Discord NullVoxPopuli let me know that "non-embroider sourcemaps are kinda borked" and that "the solution is embroider" (https://discord.com/channels/480462759797063690/1237817026899349575/1237826401386631278).

I'm guessing the team is already aware that hifi sourcemaps are impossible to achieve with the defaults that come with a new app, but I can't find any mention of that anywhere on the public web, so I'm mainly opening this issue as a way for other people to be able to find this info.

---

Output from `ember version --verbose && npm --version && yarn --version`:
```
ember-cli: 5.8.1
node: 18.19.1
acorn: 8.10.0
ada: 2.7.2
ares: 1.20.1
base64: 0.5.0
brotli: 1.0.9
cjs_module_lexer: 1.2.2
cldr: 43.1
icu: 73.2
llhttp: 6.1.0
modules: 108
napi: 9
nghttp2: 1.57.0
nghttp3: 0.7.0
ngtcp2: 0.8.1
openssl: 3.0.13+quic
simdutf: 3.2.18
tz: 2023c
undici: 5.28.3
unicode: 15.0
uv: 1.44.2
uvwasi: 0.0.19
v8: 10.2.154.26-node.28
zlib: 1.2.13.1-motley
os: darwin arm64
10.2.4
1.22.22
```