buffer: `foldmethod` is used as a buffer option

[Milly]

Describe the bug

'foldmethod' is "local to window" option.

						*'foldmethod'* *'fdm'*
'foldmethod' 'fdm'	string (default: "manual")
			local to window
			{not available when compiled without the |+folding|
			feature}
	The kind of folding used for the current window.  Possible values:
	|fold-manual|	manual	    Folds are created manually.
	|fold-indent|	indent	    Lines with equal indent form a fold.
	|fold-expr|	expr	    'foldexpr' gives the fold level of a line.
	|fold-marker|	marker	    Markers are used to specify folds.
	|fold-syntax|	syntax	    Syntax highlighting items specify folds.
	|fold-diff|	diff	    Fold text that is not changed.

But it is used as "local to buffer" option in the buffer module.

deno-denops-std/buffer/buffer.ts

Line 60 in 016970e

let foldmethod = getbufvar(a:bufnr, '&foldmethod')

deno-denops-std/buffer/buffer.ts

Line 526 in 016970e

op.foldmethod.getBuffer(denops, bufnr),

Expected behavior

Use as "local to window" option, or remove it if unnecessary.

[Milly]

@lambdalisue
What happens if we don't change the foldmethod=manual? I couldn't figure it out from the tests.

[lambdalisue]

To be honest, I don’t remember at all. But I have a vague feeling that I might have been tweaking foldmethod by mistake instead of undolevel (if so, it make sense to keep undo history.)

So, let's remove that and see what happens (we'll see.)

[Milly]

Closes by #271