docs: add jsr and experimental to Deno command

[MikeMcC399]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

It corrects the non-working Deno installation instruction in the README > Installation section.

What changes did you make? (Give an overview)

Installation instruction corrected to read as follows:

deno add jsr:@eslint/css

Deno is marked as experimental.

Related Issues

• closes Bug: Deno install error: @eslint/css is missing a prefix #66

Is there anything you'd like reviewers to focus on?

• This duplicates fix: correct deno install #70. Only one of these PRs is needed.

[nzakas]

LGTM. Thanks.

[MikeMcC399]

CI reported one error when merging this PR

https://github.com/eslint/css/actions/runs/13550339858/job/37872191467 in Verify JSR Publish

It looks like [email protected] does not like the css-tree

> @eslint/[email protected] test:jsr
> npx jsr@latest publish --dry-run

npm warn exec The following package was not found and will be installed: [email protected]
Downloading JSR release binary...
Download completed
Checking for slow types in the public API...
error[unsupported-ambient-module]: found an ambient module, which is a global augmentation, which are not unsupported
 --> /home/runner/work/css/css/typings/css-tree/index.d.ts:1:16
  | 
1 | declare module "css-tree" {
  |                ^^^^^^^^^^
  = hint: remove the ambient module declaration

I won't submit an issue for this, as it should be resolved by the updates you have already planned.