This PR adds support for file exclusion into `.gitea/template`
Docs: https://gitea.com/gitea/docs/pulls/470
### Usage
Template owners add a `.gitea/template` file to their template
repository:
```
# Files to include for variable expansion
*.go
text/*.txt
**/modules/*
# Files/directories to exclude from the generated repo
[exclude]
vendor/*
node_modules/
src/build/
```
Resolves#37202
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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Paarth