FIX #2719: enforce name argument of migration generate command

[akosasante]

User error sometimes leads to running the migration generate command with invalid parameters. If the -n argument for the name of the migration class is completely missing, it returns a useful error to the user:

> typeorm migration:generate -f ormconfig -c development
> Generates a new migration file with sql needs to be executed to update schema.

Options:
  -h, --help        Show help                                          [boolean]
  -c, --connection  Name of the connection on which run a query.
                                                            [default: "default"]
  -n, --name        Name of the migration class.                      [required]
  -d, --dir         Directory where migration should be created.
  -f, --config      Name of the file with connection configuration.
                                                          [default: "ormconfig"]
  -v, --version     Show version number                                [boolean]

Missing required argument: n

However, if the user includes the -n argument but forgets to pass in a value for that option, currently the yargs library is interpreting that as a positional argument, with a value of true (https://github.com/yargs/yargs/blob/master/docs/advanced.md#positional-arguments). This then results in the aforementioned TypeError: str.replace is not a function error when name is passed into the camelCase method.

{
  _: [ 'migration:generate' ],
  f: 'ormconfig',
  config: 'ormconfig',
  c: 'development',
  connection: 'development',
  n: true,
  name: true,
  '$0': 'node_modules/.bin/typeorm'
}
boolean
Error during migration generation:
TypeError: str.replace is not a function
    at Object.camelCase (/Users/aasante/h-dev/TradeMachine/trade-machine-server/node_modules/typeorm/util/StringUtils.js:12:16)
    at Function.MigrationGenerateCommand.getTemplate (/Users/aasante/h-dev/TradeMachine/trade-machine-server/node_modules/typeorm/commands/MigrationGenerateCommand.js:168:48)
    at Object.<anonymous> (/Users/aasante/h-dev/TradeMachine/trade-machine-server/node_modules/typeorm/commands/MigrationGenerateCommand.js:117:64)
    at step (/Users/aasante/h-dev/TradeMachine/trade-machine-server/node_modules/tslib/tslib.js:136:27)
    at Object.next (/Users/aasante/h-dev/TradeMachine/trade-machine-server/node_modules/tslib/tslib.js:117:57)
    at fulfilled (/Users/aasante/h-dev/TradeMachine/trade-machine-server/node_modules/tslib/tslib.js:107:62)
    at processTicksAndRejections (internal/process/task_queues.js:93:5)

The goal of this PR is to improve the messaging returned to the user in this case; hopefully making it more obvious how to resolve.

Previous work was done in this area to return a message if the name argument is empty: e68538a
but it doesn't work here because of the argument being coerced into a boolean value.

This PR adds the type: "string" to the name argument options passed to yarg. This tells the yarg library to treat this argument as a string at all times, and a missing value will be interpreted as an empty string, rather than a positional argument. With this PR we get the following:

{
  _: [ 'migration:generate' ],
  f: 'ormconfig',
  config: 'ormconfig',
  c: 'development',
  connection: 'development',
  n: '',
  name: '',
  '$0': 'node_modules/.bin/typeorm'
}
Please specify a migration name using the `-n` argument

Note to maintainers: I am happy to add tests if wanted; but I wasn't able to find any existing tests for what's returned by the various CLI commands; and this was a fairly small change.

Closes (or at least, resolves the confusion brought up by): #2719, #4798, #4805

[pleerock]

Looks good, thank you for your contribution!