Default mappings for gI: goToImplementation instead enters insert mode at start of line

Is your feature request related to a problem? Please describe.

The current default keybinding for gI ("enters insert mode at start of line") prioritizes a niche Vim command over goToImplementation, a fundamental code navigation feature used by a much larger majority of developers.

Describe the solution you'd like

The goToImplementation command should be treated as a primary feature with a better default shortcut. I suggest assigning it to gI. For inspiration on sensible defaults that cater to a modern developer workflow, I recommend looking at highly popular Neovim distributions like LazyVim.

Describe alternatives you've considered

A setting could be introduced in settings.json, such as "vim.mappingsMode": "IDE" (default) or "vim.mappingsMode": "vim", to let users choose their preferred keybinding philosophy. While I know it's easy to create custom mappings, I believe the default configuration should be more sensible out-of-the-box for both general developers and existing Vim users.

Additional context

The goToImplementation command is a cornerstone of modern software development for navigating any non-trivial codebase. The existing gI functionality can be easily replicated in Vim by simply using 0i. This makes gI an ideal and available candidate for a goToImplementation mapping, providing a better out-of-the-box experience for most users.

Here's the PR. Mappings works, but the tests are failing. The old tests need to be fixed, and new ones are needed for the gI mapping.

Start with PR #9688 and inspect the failing tests related to the gI mapping. Update the old expectations and add coverage for goToImplementation on gI; done means the mapping behavior and test suite pass.