refactor: use strings.Builder to improve performance

[promalert]

strings.Builder has fewer memory allocations and better performance.
More info: golang/go#75190

[Copilot]

Pull Request Overview

This PR optimizes string concatenation operations by replacing inefficient += concatenation with strings.Builder, which is more memory-efficient and performs better for multiple string concatenations.

• Replaces string concatenation using += operator with strings.Builder methods
• Updates multiple functions across the codebase to use the builder pattern
• Maintains the same functionality while improving performance

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
src/util/util.go	Updates RepeatToFill and ToKebabCase functions to use strings.Builder
src/tmux.go	Refactors runTmux to build command arguments string with strings.Builder
src/terminal.go	Converts extractPassThroughs and parsePlaceholder to use strings.Builder
src/options.go	Updates nthTransformer closure to use strings.Builder for building result strings

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[promalert]

@junegunn Hi, Could you please review this PR at your convenience? Thank you very much.