Add linting, pre-commit, and CI

[jiuguangw]

This PR provides several infrastructure related improvements to Meshcat, primarily in the form of a linter, according to the JavaScript Standard Style. A bulk of the linter errors have been addressed already, with the exception of the following (disabled via comment in src/index.js):

• camelcase
• eqeqeq
• no-undef
• no-eval
• no-redeclare
• no-var
• no-unused-expressions

In addition,

• Add pre-commit hook for the linter, which can be run by pre-commit run --all-files
• Add a GitHub Actions workflow to enforce pre-commit (triggered on PR and push to master)
• Update .gitignore
• Update Readme

Please note that GitHub repos have a setting called "run workflows from fork pull requests" - it seems that this is disabled right now, which is why the CI is not triggering. The CI result from my fork can be found here.

[jiuguangw]

@RussTedrake , just tagging you on this PR in advance of our meeting tomorrow.