Add diff options as icon buttons

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Overview

Adds toolbar controls to the right side of the diff file header for common diff display options, with matching entries in Preferences -> Code.

Toolbar

• Word wrap (toggle) — existing preference; new shortcut in the header (still available from the diff context menu and Preferences).
• Formatting marks (toggle) — show tab and space characters in the diff text.
• Comparison method (menu) — choose how Git compares lines when showing a patch in the diff view.

Preferences

New settings under Code:

• Show formatting marks (tabs and spaces)
• Comparison method (radio list, same four modes as the header menu)

(Note: Word wrap was already in Preferences, no change to this)

Comparison method

Controls how git diff is run when loading a patch for the diff view. Options map to Git’s built-in flags:

Option	Git flags
Recognize line endings and white space differences (this is the default)	None
Ignore line ending differences	--ignore-cr-at-eol
Ignore line ending and white space length differences	--ignore-cr-at-eol, --ignore-space-change
Ignore line ending differences and all white space differences	--ignore-cr-at-eol, --ignore-all-space

Note: Comparison method affects how patches are displayed on-screen only. This does not affect exporting patches, reverting files, etc. This still uses the precise methods.

Changing the comparison method reloads the current patch (re-runs git diff). Word wrap and formatting marks apply immediately without reloading.

Icons

Notes about icons I have used: All icons are MIT Licensed according to these pages:
Update: See comments below.

https://www.svgrepo.com/svg/361077/clear-all
https://www.svgrepo.com/svg/361075/circle-slash
https://www.svgrepo.com/svg/361072/circle-large-filled
https://www.svgrepo.com/svg/361073/circle-large-outline
https://www.svgrepo.com/svg/361403/whitespace
https://www.svgrepo.com/svg/361405/word-wrap

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Reference link: https://git-scm.com/docs/diff-options#Documentation/diff-options.txt-ignore-space-at-eol

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License update: I found the proper reference repo for the icons:
Reference links:
https://github.com/microsoft/vscode-codicons
https://microsoft.github.io/vscode-codicons/dist/codicon.html

License:
https://github.com/microsoft/vscode-codicons/blob/main/LICENSE

While the license for the code in the repo is MIT, the icons are under The license is Creative Commons Attribution 4.0 International. Not great. Not terrible, I guess.