feat(civil): make civil a standalone module

Fixes #6275

Author: monstermunchkin

civil/README.md

@@ -0,0 +1,68 @@
+# Civil Time
+
+[![Go Reference](https://pkg.go.dev/badge/cloud.google.com/go/civil.svg)](https://pkg.go.dev/cloud.google.com/go/civil)
+
+Go package for civil time: a time-zone-independent representation of time that
+follows the rules of the proleptic Gregorian calendar with exactly 24-hour days,
+60-minute hours, and 60-second minutes.
+
+## Install
+
+```bash
+go get cloud.google.com/go/civil
+```
+
+## Stability
+
+The stability of this module is indicated by SemVer.
+
+## Example
+
+```go
+import (
+    "fmt"
+    "time"
+
+    "cloud.google.com/go/civil"
+)
+
+// Working with dates
+d := civil.Date{Year: 2024, Month: time.March, Day: 15}
+fmt.Println(d)                    // 2024-03-15
+fmt.Println(d.AddDays(10))        // 2024-03-25
+fmt.Println(d.Weekday())          // Friday
+
+parsed, _ := civil.ParseDate("2024-03-15")
+fmt.Println(parsed == d)          // true
+
+today := civil.DateOf(time.Now())
+fmt.Println(today.After(d))       // depends on current date
+
+// Working with times
+t := civil.Time{Hour: 14, Minute: 30, Second: 0}
+fmt.Println(t)                    // 14:30:00
+
+// Working with datetimes
+dt := civil.DateTime{Date: d, Time: t}
+fmt.Println(dt)                   // 2024-03-15T14:30:00
+
+// Convert to time.Time in a specific location
+loc, _ := time.LoadLocation("Europe/Berlin")
+tt := dt.In(loc)
+fmt.Println(tt)                   // 2024-03-15 14:30:00 +0100 CET
+```
+
+## Go Version Support
+
+See the [Go Versions Supported](https://github.com/googleapis/google-cloud-go#go-versions-supported)
+section in the root directory's README.
+
+## Contributing
+
+Contributions are welcome. Please, see the [CONTRIBUTING](https://github.com/GoogleCloudPlatform/google-cloud-go/blob/main/CONTRIBUTING.md)
+document for details.
+
+Please note that this project is released with a Contributor Code of Conduct.
+By participating in this project you agree to abide by its terms. See
+[Contributor Code of Conduct](https://github.com/GoogleCloudPlatform/google-cloud-go/blob/main/CONTRIBUTING.md#contributor-code-of-conduct)
+for more information.

civil/go.mod

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+module cloud.google.com/go/civil
+
+go 1.25.0
+
+require github.com/google/go-cmp v0.7.0

civil/go.sum

@@ -0,0 +1,2 @@
+github.com/google/go-cmp v0.7.0 h1:wk8382ETsv4JYUZwIsn6YpYiWiBsYLSJiTsyBybVuN8=
+github.com/google/go-cmp v0.7.0/go.mod h1:pXiqmnSA92OHEEa9HXL2W4E7lf9JzCmGVUdgjX3N/iU=