fix: parser hangs when a bracket isn't closed (#1005)

[harrisbisset]

If a bracket (curly or parentheses) isn't closed, the parser keeps reading until it is closed.
However, the parser NEVER checks if EOF is reached. So, the parser loops when EOF and there's a missing bracket.

Solution:

Check for EOF before handling anything else (could be elsewhere, but I think this makes the most sense).
If the parser is expecting nested functions, then it should return an error with the unclosed bracket.

Example:

Templ:

package example

templ Show() {
    @Exists(
}

Result:

(✗) Error [ error=failed to generate code for "path_here/example.templ": /path_here/example.templ parsing error: templ element: invalid go expression: unbalanced '(': line 3, col 5 ]
(✗) Command failed: generation completed with 1 errors

Closes #1005

[a-h]

Thanks for your contribution, much appreciated.