Fix alias emitted in mapping

[ccoVeille]

This partially rolls back the changes made with 926dd0b with #47

The colon was allowed in the characters allowed for anchor and alias
name. But it leads to ambiguity in the YAML structure.

This PR also makes sure to emit a space when an alias is used in a key of a mapping node.

Related to:

• bug: Fix error unmarshalling alias/anchor with non alphanumeric name #47
• Anchor map key with no space before colon works in v3; breaks in v4 #109

The issue appeared with the refactoring made with #47 that made sure to respect the YAML specification that allows having a : in an anchor and alias name.

The issue was reported by @mikefarah when he tried to migrate on go.yaml.in/yaml/v4

[ccoVeille]

I should have mentionned it. This is draft. As a proof of concept and for discussion purpose.

[ccoVeille]

cc @mikefarah @inteon

[ingydotnet]

I would prefer that we revert allowing : in anchors.
The libyaml family has always been overly strict about chars allowed in anchors.
They only allow [-\w] chars.
We allowed more chars (all the 7bit ascii allowed chars) but we are currently far from spec compliant.
The spec allows all unicode chars minus the syntax chars it disallows.
I feel this is both unimportant and potentially dangerous.
Removing : will not bother anyone and will cause less problems (like the one yq reported).

[Copilot]

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