feat(markdown): add '?plain=1' for markdown files to link to code instead of preview

[linrongbin16]

Regression Test

Platforms

• windows
• macOS
• linux

Tasks

• Use <leader>gl to copy git link.
• Use <leader>gL to open git link in browser.
• Copy git link in a symlink directory of git repo.
• Copy git link in an un-pushed git branch, and receive an expected error.
• Copy git link in a pushed git branch but edited file, and receive a warning says the git link could be wrong.

[codecov]

Codecov Report

Attention: 16 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
lua/gitlinker.lua	63.07% <0.00%> (-9.66%)	⬇️

... and 3 files with indirect coverage changes

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