Support template into_toml members (Issue #255)

[CDK6182CHR]

Add a has_template_into_toml_member trait to support constructing or assigning to toml::basic_value with template into_toml method, as in the doc and Issue #255.

[ToruNiina]

The code looks perfect!

Although CI does not run here (maybe because the last commit is auto-update of single-include with [skip ci] keyword), it seems to pass the CI tests in your repo (https://github.com/CDK6182CHR/toml11/actions/runs/10017135670). We can run CI here by pushing an empty commit to your repo, but I think we don't need it because ci scripts have no change.