AppImage package support

[KRTirtho]

Requirements:

• works only in Debian/Ubuntu based Linux OS
• appimage-builder should be installed on the system & available in the PATH
• configuration file inside project's linux/packaging/appimage/make_config.yaml directory

Config file structure in linux/packaging/appimage/make_config.yaml with Examples

# application id with organization domain
appId: org.leanflutter.examples.hello_world

# relative path to icon
icon: assets/logo.png

# scripts that are run before `appimage-builder` starts to build the AppImage
script:
  - echo 'Running a Script before building'

# include direct/indirect `apt` dependencies
# if your package's dependencies are from other/unofficial/3rd-party PPA(s)/repository then add
# those repos/ppa before running the flutter distributor package
include:
  - libkeybinder-3.0

# exclude certain non-required indirect dependencies that might overweight the bundle
exclude: []

# some dependencies are available in every system & bundling them into the
# AppImage only make the bundle beefy thus some of them are excluded by default.
# It can be turned on/off with following field
# @default - true
default_excludes: true

files:
  # include extra files & folders
  include:
    - ./example.png

  # exclude files & folders
  exclude: []
  # to whether exclude opinionated non-required files by default or not  
  # @default - true
  default_excludes: true

[KRTirtho]

@lijy91 Changes made according to your suggestions