fix(url): fix url encoding issue

[linrongbin16]

Fix #224 .

I add an example that a file path contains multiple whitespaces here: https://github.com/linrongbin16/gitlinker.nvim/blob/4ce2bd4ceecf22393226f62b77074913f6aa4209/minimal_init/test%20cases/this%20is%20a%20file%20contains%20whitespaces.txt?plain=1#L1

The thing is:

1. The generated url is not encoded in URL format.
2. If open this url (with GitLink!) in browser, the browser (for my, it's google chrome) will auto encode the url into URI format, so it's still working for a browser.

Regression Test

Platforms

• [ ] windows
• [x] macOS
• [ ] linux

Hosts

• [x] Test on github.com.
• [ ] Test on gitlab.com.
• [ ] Test on bitbucket.org.
• [ ] Test on codeberg.org.
• [ ] Test on git.samba.org.

Functions

• [x] Use GitLink(!) to copy git link (or open in browser).
• [ ] Use GitLink(!) blame to copy the /blame link (or open in browser).
• [ ] Use GitLink(!) default_branch to open the /main//master link in browser (or open in browser).
• [ ] Use GitLink(!) current_branch to open the current branch link in browser (or open in browser).
• [ ] Copy git link in a symlink directory of git repo.
• [ ] Copy git link in an un-pushed git branch, and receive an expected error.
• [ ] Copy git link in a pushed git branch but edited file, and receive a warning says the git link could be wrong.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 77.61%. Comparing base (0ff97f4) to head (24569e9).

Files	Patch %	Lines
lua/gitlinker.lua	0.00%	2 Missing :warning:
lua/gitlinker/linker.lua	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #226 +/- ## ========================================== + Coverage 73.81% 77.61% +3.79% ========================================== Files 9 9 Lines 802 804 +2 ========================================== + Hits 592 624 +32 + Misses 210 180 -30 ```

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