Add .gitignore for MoonBit

[yufeiminds]

Link to the application or project's homepage

MoonBit Programming Language

Reasons for making this change

MoonBit is an programming language supporting WebAssembly, JavaScript, and native backends. It has been officially recognized by GitHub as a programming language and is gaining adoption in the open-source community.

Adding a MoonBit-specific .gitignore template will help developers avoid accidentally committing build artifacts, temporary files, and other generated content to their repositories. The build system produces outputs in the _build/ directory, and there are other language-specific files that should not be tracked by Git.

Links to documentation supporting these rule changes

Pattern	Description	Documentation Source
_build/	Default build output directory, removed by moon clean	Command-Line Help for moon
.mooncakes/	Local package cache directory	Mooncakes Registry
moonbit-coverage-*.txt	Coverage reports generated by MoonBit tooling	Measuring code coverage
_coverage	Coverage output directory (HTML reports)	Ibid.
bisect.coverage	Bisect runtime coverage data file	Ibid.
*.js.map	Source map files for JavaScript output	Foreign Function Interface (FFI)
node_modules/	Node.js dependencies	Standard directory for Node.js dependencies used in JS target workflows
*.wasm	Compiled WebAssembly binaries	Well-known WebAssembly format
*.wat	WebAssembly text format output	Command-Line Help for moon
.moonagent/	MoonBit agent runtime state and temporary files	Getting Started
*.ll	LLVM textual intermediate representation (IR)	Announcing LLVM backend for MoonBit
*.bc	LLVM bitcode files	Ibid.
*.o	Native object files from LLVM/native compilation	Ibid.
*.s	Generated assembly source files	Ibid.
*.out	Default executable output on Unix-like systems	Ibid.
*.exe	Native executable output on Windows	Ibid.
*.dSYM/	macOS debug symbol bundles	Command-Line Help for moon (see -g, --debug flag)

Merge and Approval Steps

• I have read the contribution guidelines and understand my PR will be closed if it doesn't meet these guidelines

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