Rename canary command to prerelease; simplify ChangeType

[Copilot]

Reworks prerelease publishing per the revised plan: renames the canary command, removes overlapping options, and shrinks the ChangeType union.

Command and option changes

• beachball canary → beachball prerelease (no alias).
• canaryName removed; prereleasePrefix is now the suffix used by beachball prerelease (e.g. "beta", "pr30") and defaults to "prerelease".
• prereleasePrefix no longer affects bump/publish.

ChangeType shrunk to 'patch' | 'minor' | 'major' | 'none'

• Change files using premajor/preminor/prepatch are auto-coerced to major/minor/patch with a warning at read time.
• Change files using type: "prerelease" produce a hard error directing the user to recreate the file and use beachball prerelease.
• Same migration applied to --type / --dependent-change-type CLI args.

Prerelease versioning algorithm

New pure helper bump/getPrereleaseVersion.ts:

1. Strip any prerelease component from currentVersion.
2. Apply changeType (or pass through for 'none') to get the target release.
3. Use listPackageVersions to find the highest existing ${target}-${prereleasePrefix}.<n> and return <n+1> (or start from identifierBase: '0' default, '1', or false for no counter).

This replaces the old do { semver.inc(v, 'prerelease', name) } while (versionExists) loop and fixes the 0.2.0 + minor → 0.3.0-beta.0 case (issue #676).

Prerelease → release promotion (assumption E)

bumpPackageInfoVersion strips any prerelease component from the current version before applying semver.inc, so a patch change against 1.0.0-beta.0 produces 1.0.1 rather than another prerelease.

Docs

New docs/cli/prerelease.md; sidebar updated; --prerelease-prefix rows removed from bump.md/publish.md; prereleasePrefix row in configuration.md rewritten; "Prereleases" subsection added to change-types.md.