encoding/json/v2: rename `inline` tag option as `embed`

The `inline` tag option is already used in existing Go code
for purposes outside of what the "encoding/json" has provided.
Adding support for it now unfortunately changes the semantics,
which may existing code.

Instead of the `inline` tag option, rename it as `embed`.
This better matches the Go language feature of embedding,
for which this is a logical extension of the same concept.

Analyis of the Go module proxies shows that
there are zero usages of `embed` within `json` tags.
In contrast there are ~649k occurences (0.5%) of the `inline` tag
out of a total of ~119M `json` field tags.

Existing code can migrate by adding both `inline` and `embed`,
then update to the latest version of the v2 module,
and then later remove the use of `inline`.

Fixes #79985

Change-Id: Ia695ce5197cdb9482aa55ceaa7f4430d93adb0f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/791440
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Chris Hines <chris.cs.guy@gmail.com>
Reviewed-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>