Fixes #27605: inline math blocks can't be preceeded/followed by alphanumerical characters (#30175) (#30251)

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>

modules/markup/markdown/markdown_test.go

@@ -511,9 +511,17 @@ func TestMathBlock(t *testing.T) {
 			`\(a\) \(b\)`,
 			`<p><code class="language-math is-loading">a</code> <code class="language-math is-loading">b</code></p>` + nl,
 		},
+		{
+			`$a$.`,
+			`<p><code class="language-math is-loading">a</code>.</p>` + nl,
+		},
+		{
+			`.$a$`,
+			`<p>.$a$</p>` + nl,
+		},
 		{
 			`$a a$b b$`,
-			`<p><code class="language-math is-loading">a a$b b</code></p>` + nl,
+			`<p>$a a$b b$</p>` + nl,
 		},
 		{
 			`a a$b b`,
@@ -521,7 +529,15 @@ func TestMathBlock(t *testing.T) {
 		},
 		{
 			`a$b $a a$b b$`,
-			`<p>a$b <code class="language-math is-loading">a a$b b</code></p>` + nl,
+			`<p>a$b $a a$b b$</p>` + nl,
+		},
+		{
+			"a$x$",
+			`<p>a$x$</p>` + nl,
+		},
+		{
+			"$x$a",
+			`<p>$x$a</p>` + nl,
 		},
 		{
 			"$$a$$",