process: refactor the bootstrap mode branching for readability

This patch refactors the branches for choosing the mode to run
Node.js in `internal/bootstrap/node.js`. Instead of inlining the
decision making all in `startup`, we create a `startExecution()`
function which either detects and start the non-user-code mode,
or prepares for user code execution (worker setup, preloading modules)
and starts user code execution.
We use early returns when we decide the mode to run Node.js in for fewer
indentations and better readability.

This patch also adds a few comments about the command-line switches
and a few TODOs to remove underscore properties on `process` that
are mainly used for bootstrap mode branching. It also includes
a few other refactoring such as inlining functions/variables
that are not reused and removing the default argument of
`evalScript` for better clarity.

PR-URL: https://github.com/nodejs/node/pull/24673


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>