Expose getStringLiteralType and getNumberLiteralType on the TypeChecker, plus remove /** @internal */ from several useful methods.

[sstchur]

Expose the following on the TypeChecker:

• getStringType
• getStringLiteralType
• getNumberType
• getNumberLiteralType
• getBooleanType
• getFalseType
• getTrueType
• getUndefinedType
• getNullType
• isTypeAssignableTo

Fixes #50694

I opened a suggestion issue #50694 to suggest exposing getStringLiteralType and getNumberLiteralType. The rationale was that they would help avoid questionable and hacky code in an ES-Lint rule I was working on and that they would be useful to other developers using the TypeChecker. All of the other methods were technically available on the TypeChecker, but not "visibile" due to having been marked /** @internal */. This PR exposes the two recommended in #50694 and removes the Internal designation on several others that are useful for developers leveraging the TypeChecker.

@andrewbranch suggested that I create the PR myself, so I did :-)

I read the Contributing.md, and I'm aware of the following:

Features (things that add new or improved functionality to TypeScript) may be accepted, but will need to first be approved (labelled "help wanted" or in the "Backlog" milestone) by a TypeScript project maintainer in the suggestion issue. Features with language design impact, or that are adequately satisfied with external tools, will not be accepted."

And I'm also aware that I don't have such a label from a TS project maintainer, but at Andrew's suggestion, I have created the PR anyway. Please let me know if there is anything else I can do to make the PR better or the review process easier.

[typescript-bot]

The TypeScript team hasn't accepted the linked issue #50694. If you can get it accepted, this PR will have a better chance of being reviewed.

[jakebailey]

See also #9879 (since this PR seems to contain a partial export of what that proposal suggests should be exported). @weswigham

[sstchur]

See also #9879 (since this PR seems to contain a partial export of what that proposal suggests should be exported). @weswigham

If there is broad agreement on the TS team that everything in that proposal should be exposed, I could take a crack at handling all of it. LMK.

[jakebailey]

If there is broad agreement on the TS team that everything in that proposal should be exposed, I could take a crack at handling all of it. LMK.

I'm not aware of any quite yet, but it did come up today in the context of #52467 and #52470.