Add global time scaling (#5752)

# Objective

- Make `Time` API more consistent.
- Support time accel/decel/pause.

## Solution

This is just the `Time` half of #3002. I was told that part isn't controversial.

- Give the "delta time" and "total elapsed time" methods `f32`, `f64`, and `Duration` variants with consistent naming.
- Implement accelerating / decelerating the passage of time.
- Implement stopping time.

---

## Changelog

- Changed `time_since_startup` to `elapsed` because `time.time_*` is just silly.
- Added `relative_speed` and `set_relative_speed` methods.
- Added `is_paused`, `pause`, `unpause` , and methods. (I'd prefer `resume`, but `unpause` matches `Timer` API.)
- Added `raw_*` variants of the "delta time" and "total elapsed time" methods.
- Added `first_update` method because there's a non-zero duration between startup and the first update.

## Migration Guide

- `time.time_since_startup()` -> `time.elapsed()`
- `time.seconds_since_startup()` -> `time.elapsed_seconds_f64()`
- `time.seconds_since_startup_wrapped_f32()` -> `time.elapsed_seconds_wrapped()`

If you aren't sure which to use, most systems should continue to use "scaled" time (e.g. `time.delta_seconds()`). The realtime "unscaled" time measurements (e.g. `time.raw_delta_seconds()`) are mostly for debugging and profiling.

crates/bevy_diagnostic/src/frame_time_diagnostics_plugin.rs

@@ -35,12 +35,13 @@ impl FrameTimeDiagnosticsPlugin {
     ) {
         diagnostics.add_measurement(Self::FRAME_COUNT, || frame_count.0 as f64);
 
-        if time.delta_seconds_f64() == 0.0 {
+        let delta_seconds = time.raw_delta_seconds_f64();
+        if delta_seconds == 0.0 {
             return;
         }
 
-        diagnostics.add_measurement(Self::FRAME_TIME, || time.delta_seconds_f64() * 1000.);
+        diagnostics.add_measurement(Self::FRAME_TIME, || delta_seconds * 1000.0);
 
-        diagnostics.add_measurement(Self::FPS, || 1.0 / time.delta_seconds_f64());
+        diagnostics.add_measurement(Self::FPS, || 1.0 / delta_seconds);
     }
 }

crates/bevy_diagnostic/src/log_diagnostics_plugin.rs

@@ -85,7 +85,7 @@ impl LogDiagnosticsPlugin {
         time: Res<Time>,
         diagnostics: Res<Diagnostics>,
     ) {
-        if state.timer.tick(time.delta()).finished() {
+        if state.timer.tick(time.raw_delta()).finished() {
             if let Some(ref filter) = state.filter {
                 for diagnostic in filter.iter().flat_map(|id| {
                     diagnostics
@@ -110,7 +110,7 @@ impl LogDiagnosticsPlugin {
         time: Res<Time>,
         diagnostics: Res<Diagnostics>,
     ) {
-        if state.timer.tick(time.delta()).finished() {
+        if state.timer.tick(time.raw_delta()).finished() {
             if let Some(ref filter) = state.filter {
                 for diagnostic in filter.iter().flat_map(|id| {
                     diagnostics

crates/bevy_render/src/globals.rs

@@ -57,7 +57,7 @@ fn prepare_globals_buffer(
     frame_count: Res<FrameCount>,
 ) {
     let buffer = globals_buffer.buffer.get_mut();
-    buffer.time = time.seconds_since_startup_wrapped_f32();
+    buffer.time = time.elapsed_seconds_wrapped();
     buffer.delta_time = time.delta_seconds();
     buffer.frame_count = frame_count.0;