Implement multi-file schema handling in PSL

This commit implements multi-file schema handling in the Prisma Schema Language.

At a high level, instead of accepting a single string, `psl::validate_multi_file()` is an alternative to `psl::validate()` that accepts something morally equivalent to:

```json
{
  "./prisma/schema/a.prisma": "datasource db { ... }",
  "./prisma/schema/nested/b.prisma": "model Test { ... }"
}
```

There are tests for PSL validation with multiple schema files, but most of the rest of engines still consumes the single file version of `psl::validate()`. The implementation and the return type are shared between `psl::validate_multi_file()` and `psl::validate()`, so the change is completely transparent, other than the expectation of passing in a list of (file_name, file_contents) instead of a single string. The `psl::validate()` entry point should behave exactly the same as `psl::multi_schema()` with a single file named `schema.prisma`. In particular, it has the exact same return type.

Implementation
==============

This is achieved by extending `Span` to contain, in addition to a start and end offset, a `FileId`. The `FileId` is a unique identifier for a file and its parsed `SchemaAst` inside `ParserDatabase`. The identifier types for AST items in `ParserDatabase` are also extended to contain the `FileId`, so that they can be uniquely referred to in the context of the (multi-file) schema. After the analysis phase (the `parser_database` crate), consumers of the analyzed schema become multi-file aware completely transparently, no change is necessary in the other engines.

The only changes that will be required at scattered points across the codebase are the `psl::validate()` call sites that will need to receive a `Vec<Box<Path>, SourceFile>` instead of a single `SourceFile`. This PR does _not_ deal with that, but it makes where these call sites are obvious by what entry points they use: `psl::validate()`, `psl::parse_schema()` and the various `*_assert_single()` methods on `ParserDatabase`.

The PR contains tests confirming that schema analysis, validation and displaying diagnostics across multiple files works as expected.

Status of this PR
=================

This is going to be directly mergeable after review, and it will not affect the current schema handling behaviour when dealing with a single schema file.

Next steps
==========

- Replace all calls to `psl::validate()` with calls to `psl::validate_multi_file()`.
- The `*_assert_single()` calls should be progressively replaced with their multi-file counterparts across engines.
- The language server should start sending multiple files to prisma-schema-wasm in all calls. This is not in the spirit of the language server spec, but that is the most immediate solution. We'll have to make `range_to_span()` in `prisma-fmt` multi-schema aware by taking a FileId param.

Links
=====

Relevant issue: prisma/prisma#2377

Also see the [internal design doc](https://www.notion.so/prismaio/Multi-file-Schema-24d68fe8664048ad86252fe446caac24?d=68ef128f25974e619671a9855f65f44d#2889a038e68c4fe1ac9afe3cd34978bd).

psl/parser-database/src/walkers.rs

@@ -28,6 +28,8 @@ use schema_ast::ast::WithSpan;
 
 use crate::{ast, FileId};
 
+use crate::{ast, FileId};
+
 /// A generic walker. Only walkers intantiated with a concrete ID type (`I`) are useful.
 #[derive(Clone, Copy)]
 pub struct Walker<'db, I> {