Markers on Waveform

**Is your feature request related to a problem? Please describe.**
Unlike most video editors, Audacity has no facility to place markers on the waveform - markers that stay put in the waveform position where they are placed.

There have been many posts on the Forum over the years particularly from folk who come to Audacity from video editing - and this includes Forum elf (and Emeritus ex Audacity Team member) Greg Kozikowski

**Describe the solution you'd like**
Add with one keystroke a vertical mark that stays on the waveform and can be labeled. Common in video editors. Should be draggable but attached to the audio segment. Select between any two markers easily e.g. with double click. Snap a region to the markers. Should be implemented in addition to split lines.

Very necessary for long-form documentary editing, where you are identifying and then going back and handling many different selections from a long file. Also important if recordings are edited afterwards by others.

**Describe alternatives you've considered**
Audacity's Labels provide a workaround but are far from ideal - not easy to lock to a waveform or clip position.

**Additional context**
This has long been one of Audacity's most popular Feature requests:
https://wiki.audacityteam.org/wiki/Feature_Requests#markers

Tagging @Tantacrul as a UX/design issue