add loading from base env file

[SilentCatD]

#120

This will serve some purposes:

• Share some variables between different environments
• Add an additional fallback mechanism with the base environment file
• Override existing environment variable inside the base file with a chosen env

Priority will be as follows:

• Base env file < Env file + merge with

[Copilot]

Pull Request Overview

This PR implements support for loading variables from a base environment file in addition to the main environment file. This allows sharing common variables across different environments and provides a fallback mechanism where the main environment file can override values from the base file.

• Adds baseEnvFileName parameter to the load() method for specifying an optional base environment file
• Refactors file loading logic into a reusable _loadLinesFromFile() helper method
• Implements variable precedence where base environment variables are loaded first, then overridden by main environment file variables

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
lib/src/dotenv.dart	Core implementation of base environment file loading with proper precedence handling
example/pubspec.yaml	Adds the new base environment file asset to the example app
example/lib/main.dart	Updates example to demonstrate base environment file usage
example/assets/.env	Adds override example for base environment variable
example/assets/.base.env	Creates sample base environment file with shared variables

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[Copilot]

The precedence order is incorrect. Base environment variables should be added first, but linesFromMergeWith and linesFromFile are combined before base environment variables are processed. This means mergeWith and main file variables won't properly override base environment variables.

Suggested change

	final allLines = linesFromMergeWith..addAll(linesFromFile);
	final allLines = linesFromFile..addAll(linesFromMergeWith);

[Copilot]

The order of adding entries to _envMap is incorrect according to the documented priority. Base environment variables are added after main environment variables, but they should be added first so that main environment variables can override them.