Add ability to exclude files from context

[Bekaboo]

Codeium can make vim laggy in insert mode if a huge file (>10 MB) is opened and added as a completion context in vim. This PR adds the command :CodeiumExclude and a new buffer-local flag b:codeium_excluded to exclude specific files (e.g. large files) from codeium's context.

[CLAassistant]

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