feat: add trailing_comment_spacing option to preserve comment alignment

[sobh]

Disclaimer: This PR was authored with AI assistance.

Summary

Adds a new trailing_comment_spacing configuration option that controls whether the formatter compresses whitespace before trailing inline comments to a single space, or preserves the original spacing.

This is useful for codebases that align trailing comments for readability:

-- Before (with Compress, the default/existing behavior):
local x = 1 -- x value
local y = 2 -- y value
local longname = 3 -- long name

-- After (with Preserve):
local x = 1        -- x value
local y = 2        -- y value
local longname = 3 -- long name

Configuration

Possible values:

• Compress: (default) — matches StyLua's original behavior of replacing all whitespace before a trailing comment with a single space.
• Preserve: keeps the original spacing as written in the source.

stylua.toml:

trailing_comment_spacing = "Preserve"

CLI:

stylua --trailing-comment-spacing Preserve src/

EditorConfig:

[*.lua]
stylua_trailing_comment_spacing = preserve

Changes

• Add TrailingCommentSpacing enum to stylua_lib
• Modify format_token and load_token_trivia in src/formatters/general.rs to preserve original whitespace before trailing comments when configured
• Wire up CLI flag (--trailing-comment-spacing)
• Wire up EditorConfig property (stylua_trailing_comment_spacing)
• Document the option in the README options table
• Document all EditorConfig property mappings in the README
• Add snapshot tests

Testing

• All existing tests pass (185 total)
• Added 2 new snapshot tests covering aligned comments and varying spacing
• Manually verified with both CLI flag and stylua.toml configuration