Prerequisite to https://github.com/zed-industries/zed/pull/64257

* gpui: build inspector IDs only when the window's inspector exists,
drop the inspector maps on close, skip inspector state lookups for
absent or non-selected IDs
* gpui: create inspector element renderers per inspector instance
instead of at registration
* inspector_ui: create the project and editors lazily per inspector
session, keep the init task and subscriptions cancellable, hold the
inspector weakly from the completion provider
* lsp: bound the shutdown output drain by the same timeout as the
shutdown request
* gpui_platform: add an `inspector_render` benchmark that builds with
the inspector in or out

Mean time per rendered frame with the inspector closed; overhead is
relative to the inspector compiled out.

| Benchmark | Frame, compiled out | Frame, `main` | Frame, this PR |
Overhead on `main` | Overhead on this PR |
| --- | ---: | ---: | ---: | ---: | ---: |
| 1,000 bare divs, never opened | 3.352 ms | 3.505 ms | 3.362 ms | +153
µs (+4.57%) | +10 µs (+0.29%) |
| 1,000 bare divs, opened and closed twice | 3.349 ms | 3.508 ms | 3.364
ms | +159 µs (+4.76%) | +15 µs (+0.46%) |
| 100 bare divs, opened and closed twice | 317.0 µs | 336.3 µs | 317.6
µs | +19 µs (+6.10%) | +1 µs (+0.18%) |
| 10,000 bare divs, never opened | 56.617 ms | 58.925 ms | 56.472 ms |
+2.31 ms (+4.08%) | -145 µs (-0.26%) |
| 500 panel rows with text, hover, click, never opened | 4.173 ms |
4.451 ms | 4.179 ms | +278 µs (+6.67%) | +7 µs (+0.16%) |
| 500 panel rows with text, hover, click, opened and closed twice |
4.197 ms | 4.448 ms | 4.159 ms | +252 µs (+6.00%) | -37 µs (-0.89%) |
| 50 panel rows with text, hover, click, never opened | 523.4 µs | 543.3
µs | 522.3 µs | +20 µs (+3.80%) | -1 µs (-0.21%) |
| 500 panel rows with text, hover, click, opened and closed twice,
another window inspected | 4.191 ms | 4.469 ms | 4.176 ms | +278 µs
(+6.64%) | -15 µs (-0.35%) |
| editor, cursor moves (`crates/benchmarks`, `release-fast`) | 940.9 µs
| 943.0 µs | 942.5 µs | +2 µs (+0.23%) | +2 µs (+0.17%) |




Release Notes:

- N/A
- [GPUI] Optimize GPUI inspector's runtime cost by an order of magnitude