Implement specify_df()

[jonthegeek]

Closes #4

[Copilot]

Pull Request Overview

This PR implements a new specify_df() function that allows users to specify the desired format for data frames by combining column specifications. The implementation includes comprehensive error handling and validation utilities.

Key changes:

• Added specify_df() function for creating data frame specifications
• Implemented validation utilities for argument checking (.check_args_named(), .check_args_spec())
• Added error handling infrastructure with standardized error classes
• Updated documentation with cross-references between specification functions

Reviewed Changes

Copilot reviewed 21 out of 21 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
R/specify_df.R	Core implementation of the new specify_df() function
R/aaa-conditions.R	Error handling utilities and validation functions
tests/testthat/test-specify_df.R	Comprehensive test suite for specify_df() functionality
tests/testthat/test-aaa-conditions.R	Tests for error handling and validation utilities
R/specify_fct.R	Updated documentation to reference the new function
man/*.Rd	Generated documentation files for the new functions

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