src: let embedders supply a builtin code cache without a snapshot - #65352

An embedder that creates its contexts and Environments without Node's snapshot (its own isolate, no
EmbedderSnapshotData) compiles every builtin the bootstrap touches from source in each such process, and then
serializes a fresh code cache for each of them that only a later worker thread ever reads. This adds a way to hand
Node.js a cache built ahead of time, plus a flag to skip the runtime serialization. embedtest's start-to-exit goes
from 58 to 39 ms with a supplied cache, and the flag alone saves 6 ms; node itself is unchanged.

embedtest, process start → exit, n=40 interleaved      script: `0`              script: require http, fs/promises,
                                                                                   child_process, url, util, stream
  base → this branch, nothing supplied            57.9 → 58.1 ms   n.s.          76.0 → 76.8 ms   n.s.
  + cache from GenerateBuiltinCodeCache()              → 39.3 ms   −32 %  ***         → 47.3 ms   −38 %  ***
  + kNoHarvestBuiltinCodeCache                         → 51.5 ms   −11 %  ***         → 67.7 ms   −11 %  ***
  + both                                               → 39.5 ms   −32 %  ***         → 46.6 ms   −39 %  ***
node -e 0 / node --no-node-snapshot -e 0, base → branch    24.2 → 23.9 / 59.1 → 58.9 ms, both n.s.

With the supplied cache all 114 functions the bootstrap and NewContext() compile are accepted from it,
internal/per_context/* included (NODE_DEBUG_NATIVE=CODE_CACHE).

• node::GenerateBuiltinCodeCache(context) compiles every builtin in a context made with node::NewContext() in the
  kind of isolate the cache is for (same V8, flags and read-only snapshot) and returns id + bytes for a build step to
  embed.
• node::SetBuiltinCodeCache(entries) installs process-wide entries that every BuiltinLoader created afterwards
  starts with: each Environment's, and the loader NewContext() uses for the per-context scripts. A snapshot's entries
  still merge on top, so RefreshCodeCache() now merges with insert_or_assign instead of asserting a single call.
• ProcessInitializationFlags::kNoHarvestBuiltinCodeCache stops LookupAndCompile() from serializing a cache for
  builtins compiled without one. The default stays as it is because worker threads start from that harvested cache.

embedtest gets --builtin-code-cache-create <file>, --builtin-code-cache <file> and
--no-harvest-builtin-code-cache so the test drives all three through an embedder binary: it generates a cache in one
process, checks that another process's bootstrap and per-context scripts compile from it, and that a worker started
with and without the flag does and doesn't find a harvested cache.

Tests: the new embedding test and two cctests (RefreshCodeCache merges, the process cache seeds new loaders);
embedding, cctest and the default suite pass.

Disclosure: the code, tests, measurements and this description were written by Claude Code, directed and reviewed by @codebytere.