Changes:
- `@Available` directive now parses `introduced` field as a `VersionTriplet`
- `.stringRepresentation(precisionUpToNonsignificant:)` has been moved to `VersionTriplet`
- Modifies tests so that they compile given the above changes

The availability directive supports an "introduced" field, which accepted any string. This commit modifies it to only accept strings which can be converted into a VersionTriplet, the type which DocC uses internally for representing version information.

This is useful to be able to compare the availability from the `@Availability` directive with the platform metadata which DocC's context contains, for purposes such as determining whether a platform has been configured as being in beta.

The `.stringRepresentation(precisionUpToNonsignificant:)` functionality has been moved to `VersionTriplet`, which is a non-breaking change as this was internal API.  This is because we don't always have a `SymbolGraph.SemanticVersion` as the starting type, for example when using the `@Available` directive.

Changes:
- Sets `AvailabilityRenderItem.isBeta` when `AvailabilityRenderItem.init?(_ :current:)` is called
- Makes `AvailabilityRenderItem.isBeta`, `AvailabilityRenderItem. introduced` and `AvailabilityRenderItem.name`  into `let` properties as they are integral to availability rendering.

Unifies the behavior in `AvailabilityRenderItem` so that we always compute whether a platform is in beta, regardless of whether the availability information comes from the symbol graph or comes from the `@Available` directive.

This fixes an issue where sample code articles defined with `@Available` directives would never display a beta badge regardless of whether the platform was configured to be a beta platform.

Additionally, makes `isBeta` a `let` property to avoid any future bugs, as the code will now no longer compile unless this property is set in an initializer.

Resolves rdar://129355087.

Documents the format which is expected for the "introduced" field of the `@Available` directive.

Reintroduces `Metadata.Availability.introduced` as a `String` to avoid any API breaking changes. Deprecates this field, in preparation for removal in a future release. Values for this field are derived from `Metadata.Availability.introducedVersion`.

`Metadata.Availability.introducedVersion` is now driving argument `introduced` of the directive.

- Adds unit tests for verifying that the `introduced` field is parsed with the correct formatting rules.
- Fixes a bug where a leading or trailing full stop would be considered a valid format.
- Updates FIXMEs to contain the most up-to-date information
- Refactors MetadataAvailabilityTests slightly

Resolves rdar://129355087.

- Adds tests to verify that availability defined only in articles is correctly detected as beta depending on configuration.

Resolves rdar://129355087.

When a directive argument is not convertible to a complex type (such as VersionTriplet), it is complicated for the author to know what the valid format is.

Added a field `expectedFormat` in various places which can be used to specify a string which will describe what the expected format is.

The changes are all non-breaking because they are either additive or they modify internal API.

Added a custom expected format for `VersionTriplet` to improve the diagnostic information.

Resolves rdar://129355087.

This is a source-breaking change, so removing it.

Assignments can be written succinctly using an if-expression as of Swift 5.9 (https://github.com/swiftlang/swift-evolution/blob/main/proposals/0380-if-switch-expressions.md)

Makes the test assertions clearer by rephrasing the logic. These assertions are tricky to read because `isBeta` can be one of three values: true, false and nil.

The function is only used in this file, and not needed anywhere else. We can leave it private.

This reverts commit 568318e.

Uses `SemanticVersion` type rather than `VersionTriplet`. We have 4 types in DocC to represent a semantic version, and want to converge towards using only SemanticVersion:
swiftlang#970

Unifies the "deprecated" parameter behavior with the new "introduced" parameter behaviour, by making both of them be a `SemanticVersion` type.

Updates to mention that the `deprecated` argument is also meant to be a semantic version.