Extract rule: template-no-extra-mut-helpers

[NullVoxPopuli]

Split from #2371.

[kategengler]

I don't think this ported cleanly from https://github.com/ember-template-lint/ember-template-lint/blob/main/test/unit/rules/no-extra-mut-helper-argument-test.js

[wagenet]

This looked ok to me, but if Katie thinks there's something wrong, go with what she says :)

[kategengler]

This looked ok to me, but if Katie thinks there's something wrong, go with what she says :)

I just cannot find the rule it ported from -- I think it mutated two rules together in a weird way

[wagenet]

Claude analysis says:

ETL's no-extra-mut-helper-argument — these are ETL's valid cases:
{{my-component click=(action (mut isClicked))}}
{{my-component isClickedMutable=(mut isClicked)}}
<button {{action (mut isClicked)}}>
ETL's invalid cases are all (mut x y) — two args passed directly to mut.

PR #2432's rule flags (mut this.value) — exactly the single-arg form ETL considers valid.

So three concrete issues:

1. The docs reference is wrong. The PR links to no-extra-mut-helper-argument as its source, but implements a different rule that directly contradicts it. Using both rules together would create a contradiction: ETL
   says (mut isClicked) is fine; this PR says it isn't.

2. The rule is overly broad. (mut x) is still legitimate in (action (mut x)) patterns — creating a mutable reference for classic action-based two-way binding. The PR has no exception for this, so (action (mut
   isClicked)) would be flagged despite being the explicitly-correct ETL pattern.

3. No existing rule conflict — there's no template-no-extra-mut-helper-argument in this plugin yet, so there's no naming collision. But the rule as written implements "never use (mut x) at all on a path
   expression" which is a stronger, distinct claim than what the ETL ecosystem does.

Bottom line: The rule logic is internally consistent for its stated intent ("unnecessary mut helpers"). But that intent is not sourced from ETL, the docs reference is wrong, and the rule will flag patterns ETL
explicitly endorses. It needs a clearer identity — either properly port the ETL rule (flag (mut x y)) or document this as an original, more opinionated rule with explicit rationale for why (mut x) in modern
Glimmer is always unnecessary.

[wagenet]

See comment.

[NullVoxPopuli]

marking draft, because I need to re-sync the template-lint stuff