fix(termopen): "<Esc>" to close buffer not silent

[b1nhack]

Use ":" to enter command mode, not silent by default. In addition to this, using <Cmd>...<CR> has many advantages: https://neovim.io/doc/user/map.html#%3CCmd%3E

[github-actions[bot]]

Review Checklist

Does this PR follow the Contribution Guidelines? Following is a partial checklist:

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• For example, fix(lsp): some lsp-related bugfix

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If applicable:

• [ ] Tested
  • [ ] Tests have been added.
  • [ ] Tested manually (Steps to reproduce in PR description).
• [ ] Updated documentation.
• [ ] Updated CHANGELOG.md