Fix control keys handling

[jbbarth]

Issue #: #29

Description of changes: this change switches the terminal in "raw" mode so that it handles Ctrl+key sequences better.

• IMPORTANT: I have very little experience with Go, and very little experience with low level terminal applications. I'm not sure at all this is the right fix.
• I suspect some of the stty "manual" manipulations in the codebase could be replaced by what the "term" package does, but again, I know nothing ; I tried removing them and the behaviour doesn't change
• ⚠️ the fix seems to add a new blank line after the end of the session, it's ugly but it doesn't seem to have a real effect

How I tested:

# build (I'm on a MacOS M2 so darwin-arm64)
rm -rf vendor/src/github.com/aws/session-manager-plugin/session-manager-plugin && gofmt -w src && make clean release && cp bin/darwin_arm64_plugin/session-manager-plugin ~/bin

# test before the modification ("mainline")
# => behaviour similar to homebrew installed version, Ctrl+K crashes, Ctrl+O doesn't work

# test after the modification (this PR)
aws ssm start-session --target <instance>
# basic commands
ipython
# => ctrl+O => works
# ctrl+K => doesn't kill the session
# ctrl+C, D => still working
# ctrl+J => works at first sight in the shell (new shell prompt)

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[jbbarth]

Auto-review: this could be moved to Stop() too, I don't have a strong preference. The behaviour doesn't change when I do it (we then need to add a new field originalTermState term.State on the ShellSession struct).

[jbbarth]

Auto-review: the project doesn't seem to use "go.mod", so I blindly cloned the repository:

git clone https://github.com/golang/term.git vendor/src/golang.org/x/term

... and removed the .git directory there.

[AndrewFarley]

Note: This fix works great for me and makes the "JOE" CLI editor (yes, old I know) common escape sequences to work properly and not cause random issues or shell ejection. I can't speak for the code quality, but I thumbs-up this from a functionality point of view.