Add --checkers to control number of checkers per Program

[jakebailey]

Much simplified version of #1176.

--checkers controls how many checkers are created in a Program during a non-editor compile. The default is still 4. Parse/bind/emit are still massively-concurrent.

If you pass --singleThreaded, everything is still single threaded as before; checkers will be forced to 1 and all normally concurrent operations will instead proceed in a deterministic order.

Intended to be used in tandem with a not-yet-existing --builders flag for controlling -b concurrency (probably via a revived #1692; @sheetalkamat).

[Copilot]

Pull Request Overview

This PR adds a new --checkers command-line option that allows users to configure the number of type checkers per program, providing more granular control over parallelism than the existing --singleThreaded flag.

• Introduces a new --checkers compiler option with proper parsing and validation
• Implements the logic to use the configured checker count in program initialization
• Updates baselines for command-line help output to reflect the new option

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
internal/core/compileroptions.go	Adds the Checkers field to store the configured number of checkers
internal/compiler/program.go	Implements logic to use the Checkers option with a minimum value of 1
internal/tsoptions/parsinghelpers.go	Adds parsing logic for the checkers option
internal/tsoptions/declscompiler.go	Defines the command-line option declaration for --checkers
internal/diagnostics/extraDiagnosticMessages.json	Adds diagnostic messages for the option description and default value
internal/diagnostics/diagnostics_generated.go	Generated diagnostic message constants
testdata/baselines/reference/tsc/commandLine/help-all.js	Updated baseline showing the new option in help output
testdata/baselines/reference/tsbuild/commandLine/help.js	Updated baseline showing the new option in help output