Support for Go fmt-formatted strings

[vladimirvivien]

This PR implements fmt.Sprintf functionality for all gexe methods and functions that accept string parameters.

Changes:

• Added variadic ...any parameters to all string-accepting methods on Session type
• Added corresponding updates to all package-level functions
• Maintained full backward compatibility

Fixes #70

[Copilot]

Pull Request Overview

This PR implements support for Go's fmt.Sprintf formatting across all gexe string parameters while maintaining variable expansion and backward compatibility. Key changes include:

• Adding variadic ...interface{} parameters to all string-accepting Session methods and package-level functions.
• Applying go’s fmt.Sprintf formatting conditionally with the helper function applySprintfIfNeeded.
• Updating tests and examples demonstrating sprintf formatting in various contexts.

Reviewed Changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
variables.go	Updated Session methods (SetEnv, SetVar, Eval) to support formatting
str.go	Updated String method to apply formatting
sprintf_*_test.go	Added tests to verify sprintf functionality across platforms
sprintf_helpers.go	Introduced helper functions for conditional formatting
procs.go, functions.go, filesys.go, echo.go	Updated various process and file-related functions to support formatting
examples/sprintf_demo/*	Demo application and documentation updated for sprintf support
README.md	Enhanced description of sprintf formatting support in gexe

Comments suppressed due to low confidence (3)

variables.go:69

• Update the function comment for Eval to mention the new 'args ...interface{}' parameter that allows sprintf formatting, ensuring consistency with the new API behavior.

func (e *Session) Eval(str string, args ...interface{}) string {

functions.go:60

• Revise the documentation for the Eval function to explicitly mention that it now accepts variadic formatting arguments, complementing its variable expansion functionality.

func Eval(str string, args ...interface{}) string {

echo.go:47

• Consider updating the ProgAvail comment to note that the function now supports additional variadic arguments for sprintf formatting.

func (e *Session) ProgAvail(progName string, args ...interface{}) string {