GeezerVison

Proposes two material changes to the large PR policy:

Firstly, the current line calculation (abs(added - deleted)) only really covers PRs that add or remove large featuresets. Changes to existing lines are calculated as +1/-1 by Git, so a PR that edits a million lines in-place has a "net size" of 0 and doesn't count as a large PR by this definition. A PR that makes 5,000 changes to existing code is no less challenging or time-consuming to review than one that adds 5,000 of new code, and we should include these under the policy.

This is a change from the approach discussed in #62829, so I'm anticipating that this will generate some discussion. However, the case example provided (splitting a flaky test into several subtests) would already be excluded under this policy, and 5,000 is no small threshold; it's easy to miss (or hide) something in a massive PR that purports to be doing something mundane.

Secondly, the policy wording states "add a new subsystem", but really intends to state "add a new public module"; the given example of adding a node:foo/bar module would not create a new core subsystem (it would come under foo) but should still be considered a large PR. Altered the definition accordingly.

I've also moved the paragraph that clarifies the /deps count to underneath the paragraph that it elaborates on, for greater readability.