Terminology fixes

[Technologicat]

This is getting ridiculous, but here's another PR. :)

As per https://github.com/SillyTavern/SillyTavern/issues/1739,

• Extension name is now consistently Timelines (plural!) also in the extension settings panel
• Terminology change to match ST 1.11.3: "bookmark" → "checkpoint"
  • Even the ST codebase still calls them bookmarks internally, so I changed only the user-visible strings and comments.
• ~Fix a check that didn't trigger after ST itself changed this terminology~ EDIT: Reverted as per review.
• Mention the /tl slash command in the README
• Fix whitespace in README

[Technologicat]

Thanks for the merge!