Replace shell scripts by xtask

[YgorSouza]

xtasks are effectively like shell scripts written in Rust. As they only require rustc and cargo, they solve most portability problems (see for example #2859). They also allow using any Rust crates as part of the build automation, so for example they could pull the wasm-opt crate as a dependency, instead of having this comment instructing the user to install it manually:

egui/sh/build_demo_web.sh

Lines 81 to 82 in eb0812a

	# to get wasm-opt: apt/brew/dnf install binaryen
	wasm-opt "docs/${CRATE_NAME}_bg.wasm" -O2 --fast-math -o "docs/${CRATE_NAME}_bg.wasm" # add -g to get debug symbols

So it could make the repository easier to work with for new contributors.

[emilk]

It would be nice, but there are a few concerns I have:

A) I don't want wasm-opt to be compiled in if a user calls cargo check or cargo run
B) Verbosity: rust code can become quite verbose compared to a shell script
C) Compile times: It's annoying when a simple script takes a minute to compile

All of these are probably solvable, but I wanted to flag them ahead of time