Emacs: Allow for legacy type checking in Emacs michelson-mode.

Context

When working with Michelson in Emacs, it's sometimes useful to view legacy contracts and inspect stack types just as it is when developing new smart contracts. This MR introduces an option to enable --legacy flag when type checking scripts from within michelson-mode, thus displaying a correct type information instead of an error for legacy contracts.

Manually testing the MR

1. Open a smart contract in Emacs. Initially it is type checked without --legacy flag.
2. Call michelson-legacy-mode interactive function to enable --legacy flag.
3. Call the same function again to disable it.
4. Verify that michelson-extra-flags variable can be customised and allows for adding the --legacy flag.

Checklist

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