Updated Lesson 1 project , fixed build errors and enabled flutter_lints

[chaudharydeepanshu]

Updates:

• Updated .gitignore for ios, android and flutter project.
• Replaced configuration 'compile' with 'implementation' and updated targetSdkVersion & compileSdkVersion.
• Updated kotlin version , build gradle plugin version and replaced "maven {url "https://maven.google.com"}" with "mavenCentral()".
• Updated distributionUrl in gradle-wrapper.properties.

Fixes:

• Added SDK constraint:">=2.15.1 <3.0.0" in pubspec.yaml.
• Migrated this app to the V2 embedding.
• Enabled AndroidX support.
• Fixed #8 .

Enabled flutter_lints:

• Added flutter_lints in project.
• Updated project code according to flutter_lints recommendations.