make parse_expr_with_alias public

[Eason0729]

Some very small change, as the title said.

close #1439

[alamb]

Thanks @Eason0729 -- makes sense to me

[alamb]

Could you please also add some documentation to this function (perhaps with an example or two) now that it will become part of the public API?

[Eason0729]

Sorry for missing that piece of document, I have updated it.

[coveralls]

Pull Request Test Coverage Report for Build 11091680075

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 17 of 17 (100.0%) changed or added relevant lines in 1 file are covered.
• 761 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.04%) to 89.312%

---

Files with Coverage Reduction	New Missed Lines	%
tests/sqlparser_common.rs	10	89.68%
src/ast/mod.rs	304	82.42%
src/parser/mod.rs	447	93.28%

Totals
Change from base Build 10980785660:	-0.04%
Covered Lines:	29783
Relevant Lines:	33347

---

💛 - Coveralls

[alamb]

I took the liberty of applying @iffyio 's suggestion and I think the docs are now looking very nice

Thank you @Eason0729 and @iffyio