feat: selective clean functionality

[perazz]

This PR adds new options to the fpm clean command for selectively deleting specific types of build artifacts:

• --test - Delete only test executables
• --apps - Delete only application executables
• --examples - Delete only example executables

Usage

fpm clean --test # Delete only test executables
fpm clean --apps # Delete only app executables
fpm clean --examples # Delete only example executables
fpm clean --test --apps # Delete both test and app executables

This allows to quickly clean specific build artifacts without rebuilding everything.
Resolves requirement for selective build artifact cleanup. Close #1136

[Copilot]

Pull Request Overview

This PR adds selective cleaning functionality to the fpm clean command, allowing users to delete specific types of build artifacts instead of the entire build directory. The implementation introduces new command-line flags (--test, --apps, --examples) and adds target-specific deletion logic.

• Adds selective cleaning flags for test, app, and example executables
• Refactors command-line parsing to use a shared build_settings helper
• Implements target filtering and deletion for specific artifact types

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/fpm_command_line.f90	Adds new clean flags, refactors build settings initialization, and implements validation logic
src/fpm.f90	Implements target-specific deletion functionality with scope filtering
test/cli_test/cli_test.f90	Adds test cases for the new selective clean command options

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