feat!: add flags-dir base flag

[mdonnalley]

Add --flags-dir as a base flag for all commands that extend SfCommand

Breaking change because abstract classes that extend SfCommand and define baseFlags will fail typescript compilation. Solution is to spread SfCommand.baseFlags onto your abstract class' baseFlags. Example

// src/base-command.ts
import {SfCommand, Flags} from '@salesforce/sf-plugins-core'

export abstract class BaseCommand extends SfCommand {
  public static baseFlags = {
    ...SfCommand.baseFlags,
    'global-flag': Flags.boolean()
  } 
}


// src/commands/my-command.ts
import {BaseCommand} from '../base-command.ts'
import {Flags} from '@salesforce/sf-plugins-core'

export default class MyCommand extends BaseCommand {
  public static flags = {
    'my-flag': Flags.boolean()
  }

  public async run(): Promise<void> {
    // do stuff
  }
}

Needs salesforcecli/plugin-data#864 for external nuts to pass

@W-15213828@