chore: add DENO_FUTURE env var

[littledivy]

Closes #22315

~> DENO_FUTURE=1 target/debug/deno

> globalThis.window
undefined

[bartlomieju]

Please add an integration test

[satyarohith]

maybe we should ensure that the parsed value results in boolean true?

[littledivy]

we only want to check for presence of the env. eg: this should work DENO_FUTURE=1

prior art:

deno/cli/tools/jupyter/mod.rs

Line 55 in 1ad754b

if std::env::var("DEBUG").is_ok() {

[satyarohith]

DENO_FUTURE=false will also enable future features even though user explicitly disabled it.

Looks like there is inconsistency in the codebase right now:

1. deno/cli/tools/jupyter/mod.rs

   Lines 54 to 57 in 83d72e5

   	// This env var might be set by notebook
   	if std::env::var("DEBUG").is_ok() {
   	logger::init(Some(log::Level::Debug));
   	}

2. deno/cli/lsp/language_server.rs

   Lines 1148 to 1152 in 1ad754b

   	let is_byonm = std::env::var("DENO_UNSTABLE_BYONM").as_deref() == Ok("1")
   	|| maybe_config_file
   	.as_ref()
   	.map(|c| c.has_unstable("byonm"))
   	.unwrap_or(false);

Edit: i think it's better to go with style 2 here.

[bartlomieju]

Additionally reading of env vars should be done only once in Lazy at the top level. With this setup we will read env var on each invocation of this function.

[satyarohith]

LGTM apart from the nit