Adds .editorconfig

[marcing]

To make the maintenance easier and avoid addicental tabs.

[falkenhawk]

The indent_style and indent_size settings are fine as documentation of the existing style.

However, trim_trailing_whitespace = true and insert_final_newline = true should not be enabled on a legacy codebase like this - every file touched by an editor would get reformatted on save, creating unnecessary whitespace-only diffs and messing up git blame.

Ideally we'd only trim whitespace on edited lines, but editorconfig doesn't support that - it's all-or-nothing. Both PhpStorm and VS Code have this as an IDE-level setting:

• PhpStorm: Settings → Editor → General → On Save → Remove trailing spaces on: "Modified lines"
• VS Code: files.trimTrailingWhitespaceInRegionsOnly: true

Neither IDE exposes this through editorconfig - no ij_ or vscode_ prefixed property exists for it. And trim_trailing_whitespace = true in editorconfig actually overrides the IDE's "Modified lines" preference, forcing trimming on ALL lines (IJPL-40113).

I'd suggest removing trim_trailing_whitespace and insert_final_newline, or setting them to false to explicitly prevent editors from applying whole-file reformatting.

[falkenhawk]

Also relevant: there's a long-standing open issue on the editorconfig spec itself about this exact problem - whether trim_trailing_whitespace should apply to all lines or only modified lines, with a proposal for a three-value option (modified/all/none): editorconfig/editorconfig#208

[falkenhawk]

Suggested change

	insert_final_newline = true
	trim_trailing_whitespace = true
	[*.md]
	trim_trailing_whitespace = false