[code_assets] Make LinkMode and LinkModePreference closed

[dcharkes]

Makes the link mode and link mode preference closed enums / a closed tagged union.

#3413 (comment)

Code assets changes:

• Changes the JSON schema to a closed enum for link mode preference and closed tagged union for link mode.
  • So encountering an unknown value in a json is now a parse error.
  • And will print a more elaborate error message.
• Sealed the LinkMode class.
• Skipping a version bump, we'll bump to 2.0 in the next PR due to breaking changes.

JSON syntax generator changes:

• Updated closed enum generation to pass valid enum values to _throwFormatException.
• Updated closed tagged union generationto pass valid discriminator tag values to _throwFormatException.
• Added Set<String>? expectedValues parameter to throwFormatException and errorString to format error messages as Expected one of 'a', 'b'.
• Made expectedType an optional named parameter on throwFormatException and errorString in helper_library.dart.
• Added a json path to the constructor for enums to be able to give the json path in the error message.

Other package changes:

• Update all the generated syntax (no functional change as they have no closed enums).

[github-actions]

PR Health

Breaking changes ⚠️

Package	Change	Current Version	New Version	Needed Version	Looking good?
hooks_runner	Non-Breaking	1.5.0	1.6.0-wip	1.6.0-wip	✔️
code_assets	Non-Breaking	1.2.1	1.2.1	1.3.0 Got "1.2.1" expected >= "1.3.0" (non-breaking changes)	⚠️
data_assets	None	0.20.0	0.20.0	0.20.0	✔️
hooks	Non-Breaking	2.0.2	2.1.0-wip	2.1.0-wip	✔️
record_use	Non-Breaking	0.6.0	1.0.0-wip	1.0.0-wip	✔️

This check can be disabled by tagging the PR with skip-breaking-check.

API leaks ✔️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbol	Leaking sources

This check can be disabled by tagging the PR with skip-leaking-check.

[goderbauer]

LGTM