Expose `stringify_id` at top level and add optional parameter `escape_css`

[anders-kiaer]

Closes #1625 (except the dash-docs part - but I can take that after review and if accepted).

Contributor Checklist

• [X] I have broken down my PR scope into the following TODO tasks
  • [X] Expose stringify_id at top level.
  • [X] Add css_escape function.
• [X] I have run the tests locally and they passed. (refer to testing section in contributing)
• [X] I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR

optionals

• [X] I have added entry in the CHANGELOG.md
• [ ] If this PR needs a follow-up in dash docs, community thread, I have mentioned the relevant URLS as follows
  • [ ] this GitHub #PR number updates the dash docs
  • [ ] here is the show and tell thread in Plotly Dash community