Improve library update scripts

[aheejin]

• This checks the that Emscripten directory is clean (has no uncommitted changes) before copying its contents. You can force updating with -f/--force.
• This automatically updates version numbers in readme.txt.

Suggested in #27252 (comment).

[aheejin]

@sbc100

1. Confirm the that the source directory is clean (no changes) before copying its contents

You mean to check if Emscripten directory (dest) is clean, right? Or you really meant the source (LLVM) directory should be clean? I'm not sure why that's important. Here I checked Emscripten directory. Let me know in case you meant otherwise.

2. Automatically update the readme with the commit/branch we are syncing too?

This now automatically updates version names. I didn't try to update the branch name though. This just detects emscripten-libs-NN pattern and updates the NN number, and detects x.y.z pattern to updates the full version number. I think it can be a little risky to actually get my local branch name and write it in the readme.txts, because my local branch name can be anything; it may not be emscripten-libs-22. For example, my workflow was, I cherry-picked all changes on top of emscripten-libs-22 branch, but my local branch name for this was emscripten-libs-22-downstream, because I later plan to upload this branch and make a PR against emscripten-libs-22 branch in https://github.com/emscripten-core/llvm-project. The reason i haven't uploaded that PR yet is we will have more bug fixes while testing in Emscripten.