fix: improve parsing of :nth-of-type(xn+b)

[navorite]

closes #9969

this also fixes the following along the way:
the + in nth-of-type(+xn-b) would be parsed as a combinator.

invalid cases like these are not allowed anymore:

b(+/-)b
-ax
-ax-b
-b

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[bwbroersma]

Now that I see the fix of #9969:

svelte/packages/svelte/src/compiler/phases/1-parse/read/style.js

Line 9 in 77c1fd3

const REGEX_NTH_OF = /^\s*(even|odd|(-?[0-9]*n?(\s*\+\s*[0-9]+)?))(\s*(?=[,)])|\s+of\s+)/;

I see a (valid) case of -x is also not allowed, nor +xn, which are both valid.
I did not compile it, but I would think this would fix it:

const REGEX_NTH_OF = /^\s*(even|odd|([+-]?[0-9]*n?(\s*[+-]\s*[0-9]+)?))(\s*(?=[,)])|\s+of\s+)/;

[bwbroersma]

The regex also allows for 1+1, which should not be allowed (and isn't in Svelte 4).

[navorite]

Now that I see the fix of #9969:

svelte/packages/svelte/src/compiler/phases/1-parse/read/style.js

Line 9 in 77c1fd3

const REGEX_NTH_OF = /^\s*(even|odd|(-?[0-9]*n?(\s*\+\s*[0-9]+)?))(\s*(?=[,)])|\s+of\s+)/;

I see a (valid) case of -x is also not allowed, nor +xn, which are both valid.
I did not compile it, but I would think this would fix it:

const REGEX_NTH_OF = /^\s*(even|odd|([+-]?[0-9]*n?(\s*[+-]\s*[0-9]+)?))(\s*(?=[,)])|\s+of\s+)/;

I've also noticed more problems with the original matcher.

It should fail when matching (the 2nd & 3rd line also work in svelte 4, but they should not)

b(+/-)b
-ax
-ax-b
-b

The new regex covers the correct cases only: (+/-)xn+b or -xn+b or xn-b or (+)b

So now it matches

10n+1
even
odd
-5n+3
-5n+10
10n-1
12
+2n
n+10
2n+10 of 
2n of 

but it doesn't match

1+1
1-1
-2n
-2n-3
-10
-1

[navorite]

@dummdidumm Running the parser-modern test with these lines added in the test

    h1:nth-of-type(10n+1)
    {
        background: red;
    }

    h1:nth-of-type(+10n-1)
    {
        background: red;
    }

produces incorrect results, the + in +10n-1 gets parsed as a combinator, this isn't correct, right? it should be part of the selector

we can ofcourse stop matching + at the start of the nth selector but i'd rather match all valid syntax than patching it up

[dummdidumm]

Thank you!