Add configurable concurrency option

[jakebailey]

This adds an experimental --concurrency flag. Example usage:

• --concurrency <default|auto|on|yes> - default behavior, uses 4 checkers, plus all cores for parse/bind/emit. Theoretically we could change this to some magic algorithm.
• --concurrency <single|off|no>, --singleThreaded - one checker, single threaded parse/bind/emit.
• --concurrency max - Uses GOMAXPROCS checkers, plus all cores for parse/bind/emit.
• --concurrency half - Uses GOMAXPROCS/2 checkers, plus all cores for parse/bind/emit. (For me, this option is best for checking VS Code.)
• --concurrency 10 - Uses 10 checkers, plus all cores for parse/bind/emit. Can be any number all the way up to the number of files.
• For testing, --concurrency checker-per-file creates a checker for every file; max concurrency, for testing race conditions.

Maybe this is overkill, I can drop some of this.

[Copilot]

Pull Request Overview

This PR introduces a configurable concurrency option, replacing the old single-threaded testing flag with a new --concurrency flag that allows detailed control over the number of checkers and threads used during compilation. Key changes include:

• Adding a new "concurrency" field to compiler options and command line declarations.
• Creating a new core package for parsing and handling concurrency values.
• Updating test utilities, the compiler runner, and CI configurations to support the new flag.

Reviewed Changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
internal/tsoptions/parsinghelpers.go	Added parsing for "concurrency" flag in compiler options.
internal/tsoptions/declscompiler.go	Declared the new "concurrency" option for the compiler.
internal/testutil/harnessutil/harnessutil.go	Set default concurrency from environment if not provided via flag.
internal/testrunner/compiler_runner.go	Updated concurrency condition for skipping error baselines.
internal/core/concurrency.go	Introduced parsing logic and tests for various concurrency values.
internal/compiler/program.go	Updated program initialization to use new concurrency settings.
Herebyfile.mjs	Updated environment variable usage to use "concurrency" instead of the old flag.
.github/workflows/ci.yml	Updated environment variable in CI to support the new concurrency flag.

[DanielRosenwasser]

Instead of concurrency, I would recommend checkConcurrency.

[jakebailey]

Except that this also controls the concurrency of the parse/bind/emit stages too... I wasn't happy with any name I came up with.