internal/task: rename semver to releaseVersion and add string method

1. Remove prerelease field from semver and rename it to releaseVersion.
2. parseSemver rename to parseVersion returns a releaseVersion and
   the corresponding prerelease string.
3. Refactor vscode go release process follow the same pattern as gopls.
   Move some code change from CL 612115.

Change-Id: Ic02f26c02db519e475118e80e09748c0c63d1521
Reviewed-on: https://go-review.googlesource.com/c/build/+/611975
Reviewed-by: Hyang-Ah Hana Kim <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: h9jiang