Add test job timeout.
Add Makefile target for a Bookworm Go 1.21 based dev container.
Reformat structs.go using Go 1.21.

…per RFC5322. (#48)

…labels (#54)

* [ISSUE-49] Add test cases for iso-8859-11, windows-874, and tis-620 encoding using Thai as an example.

* [ISSUE-49] Fix: modify `decoders.decodeHeader.CharsetReader` to lookup the original label, replace `windows-` with `cp` only if not found, and raise informative error, if not found again.

Bumps [golang.org/x/text](https://github.com/golang/text) from 0.12.0 to 0.13.0.
- [Release notes](https://github.com/golang/text/releases)
- [Commits](golang/text@v0.12.0...v0.13.0)

---
updated-dependencies:
- dependency-name: golang.org/x/text
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Bumps [golang.org/x/net](https://github.com/golang/net) from 0.14.0 to 0.15.0.
- [Commits](golang/net@v0.14.0...v0.15.0)

---
updated-dependencies:
- dependency-name: golang.org/x/net
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Bumps [actions/checkout](https://github.com/actions/checkout) from 3 to 4.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v3...v4)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>