Run semu inside web browser via WebAssembly translation

[ChinYikMing]

WebAssembly is capable to port rv32emu to browser. Then, I would like to test the WebAssembly translation of the semu codebase to see if kernels (like Linux or xv6) can be booted in a browser in order to facilitate the further integration of semu to rv32emu.

I would like to start with the smaller codebase and boot xv6 first. A few technical points are as follows:

1. It is necessary to simulate a new shell that is operating in a web frontend (Xterm.js is a good solution), after which the command is forwarded to the kernel's running shell. Some of the scope issues with the variables generated by Emscripten are confusing to me, so that I have implemented a hardcoded ls command to confirm the kernel is booted successfully. The demo below shows Emscripten's capability to boot a kernel.
2. The sh shell's dollar sign ($) of xv6 does not flush after exec, possibly some source kernel code needs to be changed.

Demo: xv6

You shall see output of ls command like below:

$ .              1 1 1024
..             1 1 1024
README         2 2 2059
cat            2 3 25016
echo           2 4 23800
forktest       2 5 13688
grep           2 6 28568
init           2 7 24896
kill           2 8 23752
ln             2 9 23704
ls             2 10 27320
mkdir          2 11 23856
rm             2 12 23840
sh             2 13 43824
stressfs       2 14 24864
usertests      2 15 159064
grind          2 16 39424
wc             2 17 26208
zombie         2 18 23224
console        3 19 0

Next, command bridging from web frontend shell(Xterm.js) to real shell will be solved, and then boot the Linux kernel.