Prep for release 1.2.0

[richardcocks]

Bump the package version to 1.2.0, promote the Unreleased changelog section to v1.2.0 (adding a line for the non-TTY watch fix), and refresh the playground manifest via gleam deps update vouch.

Changes going out in this release:

• Watch mode ported to the JavaScript target (Node and Deno) (Port watch mode to the JavaScript target #4)
• Inner test runs follow the watcher's own target; --target accepts both = and space spellings (Port watch mode to the JavaScript target #4)
• Jest/Vitest-style watch keys on the JavaScript host (Port watch mode to the JavaScript target #4)
• Watch mode no longer crashes when stdout is not a TTY (Fix watch mode crash when stdout is not a TTY #5)
• Internal: @target conditional compilation replaced with runtime dispatch (Replace @target conditional compilation with runtime dispatch #3)
• Test/CI: e2e port-capture race fix (Fix port-capture race behind the e2e JSON-stream flake #6), Hex badges (Add hex badges #7), CI dependency caching (Cache Hex packages and add a fallback mirror in CI #8)

After merging, push a v1.2.0 tag to trigger release.yml, which publishes from CI.

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