Fix faulty regex for syntax highlighter

[notmeegoreng]

Description

When a single | or single & (not || or &&) is used, not as the final character in the line nor in a multiline comment (but a single line comment still has the bug), the syntax highlighting fails.

Starting from

• the end of a single numeral literal at the start of the line o,
• or the latest occurrence of the character doubled (| => ||, & => &&),
• otherwise the start of the line,

it highlights the entire line up to the | and one character after in pink, the colour used for new and with.

Characters past one after the single | or & would be highlighted normally even if they are part of a comment.

We can observe this on the playground here: https://share.sourceacademy.nus.edu.sg/syntaxhighlightingbug

I identified the issue as a faulty regex here:

js-slang/src/editors/ace/modes/source.ts

Lines 116 to 118 in f6ea571

	return /\.{3}|--+|\+\++|\^|(==|!=)[^=]|[$%&*+\-~\/^]=+|[^&]*&[^&]/
	}
	return /\.{3}|--+|\+\++|\^|(==|!=)[^=]|[$%&*+\-~\/^]=+|[^&]*&[^&]|[^\|]*\|[^\|]/

The cases for '|' and '&' capture other parts of the string in the attempt to exclude '||' and '&&'.
This PR has a fix for this, using lookahead and lookbehind assertions.
This PR also includes a few lines in the test for this file to cover the case of '|'.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code quality improvements

How to test

I expanded the existing tests for this file to cover this change.
https://github.com/notmeegoreng/js-slang/blob/patch-1/src/__tests__/mode.ts#L82

Checklist

• I have tested this code

[coveralls]

Pull Request Test Coverage Report for Build 10862938302

Details

• 1 of 2 (50.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 81.61%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/editors/ace/modes/source.ts	1	2	50.0%

Totals
Change from base Build 10841419701:	0.0%
Covered Lines:	10983
Relevant Lines:	13081

---

💛 - Coveralls

[s-kybound]

Looks good to me, tested the regex online.