Support for DART WZ-S formaldehyde sensor

When researching formaldehyde sensors recently, I found that the DART WZ-S has high precision, comes pre-calibrated, and offers good accuracy.

As ESPHome now fully requires using external components, I discovered a well-developed external component code repository on Gitee. However, after placing the files according to specifications, the ESPHome Builder interface shows "Platform not found: 'sensor.dart'". I hope someone can test whether this compiles successfully and help answer my question.

Sorry if this is not the right place to post this - please let me know if I should post elsewhere.

I have uploaded the relevant files to my repository and ask the community to review them. Please help verify if these implementations correctly integrate the DART WZ-S sensor into ESPHome via external_components.

https://github.com/bbluvgg/wz-s/

`external_components:
- source:
type: local
path: my_components
components: [dart]

uart:
- id: utt2
tx_pin: 18
rx_pin: 5
baud_rate: 9600

sensor:
- platform: dart
uart_id: utt2
formaldehyde:
name: "DART Formaldehyde"
unit_of_measurement: "µg/m³"
accuracy_decimals: 2
update_interval: 20s`


Start with the linked wz-s repository and the shown external_components configuration, then reproduce the "Platform not found: 'sensor.dart'" error in ESPHome Builder. Done means determining whether the uploaded DART WZ-S component compiles and integrates successfully through external_components, or documenting the integration problem.

Written by the indexing model from the issue text.