Provide a `${this.__name__}` variable (like `${this.__env__}` but valid in all sections)

What kind of issue is this?

• Feature Request.

Configuration

PlatformIO Version (platformio --version): 6.1.6

Description of problem

As stated in https://github.com/platformio/platformio-core/issues/4480#issuecomment-1404253063, I am using ${this.__env__} to get the environment name in a non-environment section that will later be extended by an environment. This is being rejected as invalid (but it works) so I need the suggested ${this.__name__} to insert the name of the section/environment when the value is being evaluated.

[a]
test_testing_command =
	script.sh
	${platformio.build_dir}/${this.__name__}/test
	${platformio.build_dir}/${this.__name__}/program

[env:b]
extends = a

Where ${this.__name__} is expected to evaluate to b when the test_testing_command is used (this works with ${this.__env__} on 6.1.5).

Start with the configuration variable evaluation path described in the issue and compare how ${this.__env__} is handled in environment and non-environment sections. Use the [a] and [env:b] example to verify that ${this.__name__} resolves to b when the extended test command is used; done means the variable is accepted and evaluates correctly without regressing existing behavior.