Backport #30175 by @jmlt2002
- Inline math blocks couldn't be preceeded or succeeded by
alphanumerical characters due to changes introduced in PR #21171.
Removed the condition that caused this (precedingCharacter condition)
and added a new exit condition of the for-loop that checks if a specific
'$' was escaped using '\' so that the math expression can be rendered as
intended.
- Additionally this PR fixes another bug where math blocks of the type
'$xyz$abc$' where the dollar sign was not escaped by the user, generated
an error (shown in the screenshots below)
- Altered the tests to accomodate for the changes
Former behaviour (from try.gitea.io):
![image](https://github.com/go-gitea/gitea/assets/114936010/8f0cbb21-321d-451c-b871-c67a8e1e9235)
Fixed behaviour (from my local build):
![image](https://github.com/go-gitea/gitea/assets/114936010/5c22687c-6f11-4407-b5e7-c14b838bc20d)
(Edit) Source code for the README.md file:
```
$x$ -$x$ $x$-
a$xa$ $xa$a 1$xb$ $xb$1
$a a$b b$
a$b $a a$b b$
$a a\$b b$
```
Signed-off-by: João Tiago <joao.leal.tintas@tecnico.ulisboa.pt>
Co-authored-by: João Tiago <114936010+jmlt2002@users.noreply.github.com>
(cherry picked from commit ac65aeecbd42ed30cc0838b3fad12b58daadf3ad)