fix: make set_key export output shell-safe

[j0m0k0]

Summary

This makes set_key(..., export=True) safer when it writes shell-style .env lines.

When export=True is used, python-dotenv writes output intended to be usable by a shell, for example with source .env. Before this change, a value containing a single quote, or an invalid key name containing shell syntax, could make the generated file run a command when sourced.

Changes

• Validate exported keys as shell variable names.
• Quote export values from the raw value instead of an already-quoted dotenv value.
• Escape shell-sensitive characters when double quotes are needed.
• Add regression tests for unsafe values and unsafe keys.

Related

Related to #543 and #647. That PR handles values containing single quotes, but it does not cover unsafe key names. This PR also keeps the fix focused on the export=True shell-output path.