Improvements to match formatting

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As originally discussed and described in https://github.com/rust-lang/style-team/issues/196#issuecomment-2810856708, we want to do the following:

• We want to say that whenever we wrap the { to a new line, we always put the => on the same line as the { rather than hanging it on the end of the previous line. In other words, "never break between the => and the {".
• We want to break the => { onto a new line if the pattern spans multiple lines and has indented continuation lines (because it has an if guard).

(2026-03-03:) Originally, there was a third rule proposed:

• We want to always break a match arm body onto a new line if the pattern spans multiple lines (and thus always use braces for such a match arm, given the default setting of match_arm_blocks).

In https://github.com/rust-lang/rust/issues/152763#issuecomment-3992508472, we decided not to do this one as part of this proposal.

cc @rust-lang/style

Start by locating rustfmt's match-arm formatting entry point and existing tests for match formatting. Implement the two agreed rules for => { placement, while leaving the rejected third rule unchanged. Done means coverage demonstrates the required wrapping behavior for multiline patterns and if guards.