Add variable definition "def" in switch

[atkin-mc]

Add local variable definition to the message.switch statement. This follows the same rules of scoping as message.validation.error (i.e., all variables are in the same namespace regardless of scope, so it is not possible to introduce a duplicate variable name in a child scope). Variable definitions for a switch case are only evaluated when the if for the case evaluates to true.

This PR also adds test cases for switch to source/file_test.go and adds some functionality to source/file.go that was left out of #356

Also, docs/reference.md doesn't mention message.validation.error.def, message.validation.error.details.def, this PR documents those fields.

[github-actions]

Code Metrics Report

	main (6ce0d51)	#360 (bcffbf4)	+/-
Coverage	67.6%	67.9%	+0.2%
Code to Test Ratio	1:0.4	1:0.4	+0.0
Test Execution Time	7m9s	7m10s	+1s

Details

  |                     | main (6ce0d51) | #360 (bcffbf4) |  +/-  |
  |---------------------|----------------|----------------|-------|
+ | Coverage            |          67.6% |          67.9% | +0.2% |
  |   Files             |             84 |             84 |     0 |
  |   Lines             |          15424 |          15582 |  +158 |
+ |   Covered           |          10440 |          10587 |  +147 |
+ | Code to Test Ratio  |          1:0.4 |          1:0.4 |  +0.0 |
  |   Code              |          47935 |          48318 |  +383 |
+ |   Test              |          20190 |          20407 |  +217 |
- | Test Execution Time |           7m9s |          7m10s |   +1s |

Code coverage of files in pull request scope (75.3% → 75.6%)

Files	Coverage	+/-	Status
generator/code_generator.go	78.0%	+0.1%	modified
grpc/federation/cel.go	0.0%	0.0%	modified
grpc/federation/generator/decode.go	67.5%	-0.1%	modified
grpc/federation/generator/encode.go	77.1%	0.0%	modified
internal/testutil/builder.go	84.9%	+0.2%	modified
resolver/def.go	91.7%	-1.8%	modified
resolver/format.go	89.9%	+0.1%	modified
resolver/message.go	73.5%	+0.0%	modified
resolver/resolver.go	82.7%	-0.2%	modified
resolver/types.go	87.5%	0.0%	modified
source/clone.go	83.3%	0.0%	modified
source/file.go	78.9%	+1.6%	modified
source/location.go	0.0%	0.0%	modified
source/location_builder.go	77.8%	+0.3%	modified

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Reported by octocov

[goccy]

Awesome !!! LGTM 👍