chore: add basic Markdown linting config

[cowuake]

Description

Add configuration for simplified handling of Markdown (and hence repository documentation) linting.

List of proposed changes

• Add NPM dev dependency: markdownlint-cli2
• Add markdownlint-cli2 basic configuration
• Add NPM scripts to launch markdownlint-cli2 (for both linting and fixing)
• Add DavidAnson.vscode-markdownlint to the list of VSCode recommended extensions (the extension is powered by the same linting engine behind markdownlint-cli2!)
• Update VSCode settings to allow automatic linting/fixing of Markdown files on save when the DavidAnson.vscode-markdownlint extension is installed

How to test the changes

1. After updating local NPM dependencies, just run npm run mdlint for a linting dry run or npm run mdlint:fix to apply linter's suggestions. Revert the changes before proceeding to step 2.
2. After installing the above-mentioned VSCode extension, reach for a Markdown file affected by some non-conformity to the Markdown standard (e.g., the PULL_REQUEST_TEMPLATE.md file, missing a blank line at the beginning of the Modified packages section and including an exceeding blank line at the end), and save the file to observe the automatic changes.

Modified packages

• flower-core
• flower-react
• flower-demo
• flower-devtool

(The effects of applying the linter's suggestions have not been included in this pull request, so none of the packages have been effectively modified.)

[gispada]

Can we ignore changelog files? They are auto-generated, and markdownlint-cli2 is picking them adding unnecessary noise.

I also see a few node_modules (for instance packages/flower-devtool/node_modules/glob/README.md) paths when running npm run mdlint. Can you double check? Thanks.

[cowuake]

@gispada, I've excluded changelog files and node_modules directories from the linting.
Let me know whether your consider useful or not, for this PR, to further reduce noise by suppressing reports for the MD013 rule, unless we want to set a different maximum number of lines (default: 80) right now with a further change on the configuration.