[Fix #6936] Fix MultilineMethodArgumentLineBreaks when bracket hash assignment on multiple lines

[maxh]

Fixes #6936 by handling this case:

a['b'] = c(
  1,
)

The [] assignment is represented in the AST by a send node with b as an argument and the expression on the right as an argument. It's ok that these are on the same line.

---

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[mikegee]

I support merging this as is, since it definitely fixes a bug. 👍

I'm also concerned this PR allows for similar bug. Is the following code allowed incorrectly?

a['b',
  'c', 'd'] = e

[maxh]

I'm also concerned this PR allows for similar bug. Is the following code allowed incorrectly?

This PR fixes that bug as well. Added a spec to illustrate/cover that case.

[bbatsov]

Thanks!