This feature serves as an immediate fix in case
the new export approach causes issues.
The user can simply set the environment variable `ESP_IDF_LEGACY_EXPORT`,
and the old export script will be used.
Follows the approach used in Initialise-Idf.ps1 created by the tools
installer. This is the closes equivalent of "alias" used in export.bat
Closes https://github.com/espressif/esp-idf/issues/9849
Esptool.py is available from the esptool package which is installed in
every ESP-IDF v5.0+ shells. Therefore, the path to the legacy wrapper
scripts is not needed to be in the PATH. In some cases like
https://github.com/espressif/esp-idf/issues/10926 it even cases problem
because the wrapper doesn't have a shebang line and is not executable.
Note that the wrapper is kept only for invoking esptool as "python
$IDF_PATH/..../esptool.py" which was previously used mostly in CMake
files. There should be no other use-case where the esptool wrappers are
still needed.
With strict debugging, i.e. `Set-PSDebug -strict`, execution of export.ps1 will
report the following error:
```
The variable '$envars_array' cannot be retrieved because it has not been set.
At C:\path\to\export.ps1:15 char:1
+ $envars_array # will be filled like:
+ ~~~~~~~~~~~~~
+ CategoryInfo : InvalidOperation: (envars_array:String) [], RuntimeException
+ FullyQualifiedErrorId : VariableIsUndefined
```
Closes https://github.com/espressif/esp-idf/pull/7819
