[Feature Request]: Move all methods from `TealAppDriver` to function calls

### Feature description

The Methods' body on the `TealAppDriver` class should be defined as internal function calls to trigger all checks in R CMD check.

This would avoid problems such as #1197 and follow the same pattern as `shinytest2::AppDriver` source code.

```r
TealAppDriver <- R6::R6Class( # nolint: object_name.
"TealAppDriver",
inherit = shinytest2::AppDriver,
# public methods ----
public = list(
initialize = function(a, b) {
private$a <- a
private$b <- b
},
foo = function(d, e) {
private$a + self$bar() + d + e
},
bar = function() {
private$b * 2
}
),
private = list(a = 0, b = 0)
)
```

To:

```r
TealAppDriver <- R6::R6Class( # nolint: object_name.
"TealAppDriver",
inherit = shinytest2::AppDriver,
# public methods ----
public = list(
initialize = function(a, b) app_driver_initialize(self, private, a, b),
foo = function(d, e) app_driver_foo(self, private, d ,e),
bar = function() app_driver_bar(self, private)
),
private = list(a = 0, b = 0)
)

# Functions below defined on a different file (either 1 file per function or grouping related functions in a file `TealAppDriver-methods.R`)

app_driver_initialize <- function(self, private, a, b) {
private$a <- a
private$b <- b
}

app_driver_foo <- function(self, private, d, e) {
private$a + self$bar() + d + e
}

app_driver_bar <- function(self, private) {
private$b * 2
}

```

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