Closes https://github.com/zed-industries/zed/issues/38453

Current `Buffer` API only allows getting buffer text with `\n` line
breaks — even if the `\r\n` was used in the original file's text.

This it not correct in certain cases like LSP formatting, where language
servers need to have original document context for e.g. formatting
purposes.

Added new `Buffer` API, replaced all buffer LSP registration places with
the new one and added more tests.

Release Notes: 

- Fixed ESLint linebreak-style errors by preserving line endings in LSP
communication

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Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Kirill Bulatov <kirill@zed.dev>