editor: Ignore soft wrapped lines when adding selection above or below

[dinocosta]

• Move the start_of_relative_buffer_row function from the vim::motion module to the editor::display_map::DisplaySnapshot implementation as a method.
• Add skip_soft_wrap field to both AddSelectionAbove and AddSelectionBelow actions. When set to true, this will skip soft wrapped lines when extending the selections.
• Update the default behavior for both editor: add selection above and editor: add selection below commands in order to skip over soft wrapped lines by default, mirroring VS Code's default behavior.
• Update existing keymaps to specify this skip_soft_wrap value for both AddSelectionAbove and AddSelectionBelow actions.

Closes #16979

Release Notes:

• Updated both the editor: add selection above and editor: add selection below commands to ignore soft wrapped lines. If you wish to restore the old behavior, add the following to your keymap file:

  {
    "context": "Editor",
    "bindings": {
      "cmd-alt-up": ["editor::AddSelectionAbove", { "skip_soft_wrap": false }],
      "cmd-alt-down": ["editor::AddSelectionBelow", { "skip_soft_wrap": false }]
    }
  }
  

[dinocosta]

@smitbarmase I've pushed the final commit – d972f21 ✅

Feel free to take a look at the test and let me know if it looks good! I've also removed the implementation of the Default trait for both AddSelectionAbove and AddSelectionBelow, as I've confirmed that Serde's #[serde(default = ...)] is only meant for deserialization, to know which values to use if fields are not specified 👍