Add ShellEscape node for safe string escaping in shell commands

[Nickalus12]

Add a new ShellEscape node that takes a string input and produces a safe, POSIX-compliant shell-escaped output. This uses single quotes around the string and properly escapes any internal single quotes to prevent shell injection attacks in command construction (e.g., replacing Exec + Join patterns).

The implementation includes a helper for escaping and handles the empty string case. A unit test suite covers common scenarios: empty strings, spaces, single quotes, and newlines. A brief example is added to the nodes README.

Fixes #50

[vivax3794]

Did you explore much of the codebase before working on this? It seems to be putting the code in a rather strange place, and inventing imports.

If you are unsure how the codebase works feel free to ask questions here, or join the discord linked in the repo description.

[Nickalus12]

You're right, I didn't explore the codebase thoroughly enough before writing this. I see the nodes should live in serpentine/src/engine/nodes.rs rather than as a separate module. I'll rework this to follow the existing patterns. Thanks for pointing that out.