templatevars: parse CLI opts with literal_eval

[oliver-sanders]

Apologies, oversight on my part, it turned out to be a three line change rather than the two lines predicted.

For this PR I have upgraded the set-file logic along with the CLI option (hence the unexpected third line), can revert though it would be somewhat inconsistent. If it is a problem then we could potentially use a different CLI opt for specifying type variables.

Rationale (discussed at meeting)

• We would expect the -s, --set option to be used interactively so the change shouldn't impact user scripts.
• And if it does the CLI has changed anyway.
• Set-files may be more controversial, however, I don't think these are very widely used.

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Appropriate tests are included (unit and/or functional).
• Appropriate change log entry included.
• (master branch) I have opened a documentation PR at cylc/cylc-doc/pull/XXXX.
• No dependency changes.

[hjoliver]

Hahaha 🤣 Quickest PR ever.

[hjoliver]

LGTM, tested as working 👍