Add `Env::is_process_alive` and `LocalPid::is_alive` #1116

Job	Run time
Build ubuntu stable	18s
Build ubuntu nightly	19s
Format	22s
Build windows stable	1m 56s
Build windows nightly	43s
Build macos stable	26s
Build macos nightly	33s
OTP 26 / Elixir 1.16 / Rust stable / OS macos	2m 40s
OTP 26 / Elixir 1.16 / Rust stable / OS windows	3m 21s
Clippy ubuntu	18s
OTP 26 / Elixir 1.16 / Rust stable / OS ubuntu	1m 37s
Clippy windows	1m 34s
OTP 26 / Elixir 1.16 / Rust nightly / OS macos	2m 20s
Clippy macos	24s
OTP 26 / Elixir 1.16 / Rust nightly / OS windows	2m 53s
OTP 26 / Elixir 1.16 / Rust nightly / OS ubuntu	1m 35s
OTP 26 / Elixir 1.15 / Rust stable / OS macos	3m 38s
OTP 26 / Elixir 1.15 / Rust stable / OS windows	4m 11s
OTP 26 / Elixir 1.15 / Rust stable / OS ubuntu	1m 17s
OTP 26 / Elixir 1.15 / Rust nightly / OS macos	2m 14s
OTP 26 / Elixir 1.15 / Rust nightly / OS windows	3m 22s
OTP 26 / Elixir 1.15 / Rust nightly / OS ubuntu	1m 16s
OTP 25 / Elixir 1.15 / Rust stable / OS macos	1m 59s
OTP 25 / Elixir 1.15 / Rust stable / OS windows	3m 45s
OTP 25 / Elixir 1.15 / Rust stable / OS ubuntu	1m 37s
OTP 25 / Elixir 1.15 / Rust nightly / OS macos	2m 10s
OTP 25 / Elixir 1.15 / Rust nightly / OS windows	2m 45s
OTP 25 / Elixir 1.15 / Rust nightly / OS ubuntu	1m 16s
OTP 24 / Elixir 1.14 / Rust stable / OS macos	2m 43s
OTP 24 / Elixir 1.14 / Rust stable / OS windows	3m 34s
OTP 24 / Elixir 1.14 / Rust stable / OS ubuntu	1m 19s
OTP 24 / Elixir 1.14 / Rust nightly / OS macos	2m 27s
OTP 24 / Elixir 1.14 / Rust nightly / OS windows	3m 1s
OTP 24 / Elixir 1.14 / Rust nightly / OS ubuntu	1m 10s
	1h 5m 3s

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