Automatically add labels to github pull requests

[Raruto]

Related to: #120, it may be worth adding labels automatically in order to generate releases notes in a simpler and faster way:

• https://github.com/marketplace/actions/labeler
• qgis/QGIS@ef14f6a
• Add useful labels to your Pull Requests with Github Actions

Here is a fairly comprehensive list of some the labels that could be used: https://github.com/qgis/QGIS/labels

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Basic example:

# .github/labeler.yml

# Add 'label1' to any changes within 'example' folder or any subfolders
label1:
- example/**/*

# Add 'label2' to any file changes within 'example2' folder
label2: example2/*

# Add 'repo' label to any root file changes
repo:
- '*'

# Add '@domain/core' label to any change within the 'core' package
@domain/core:
- package/core/*
- package/core/**/*

# Add 'test' label to any change to *.spec.js files within the source dir
test:
- src/**/*.spec.js

# Add 'source' label to any change to src files within the source dir EXCEPT for the docs sub-folder
source:
- any: ['src/**/*', '!src/docs/*']

# Add 'frontend` label to any change to *.js files as long as the `main.js` hasn't changed
frontend:
- any: ['src/**/*.js']
  all: ['!src/main.js']