Optional treesitter

[radbasa]

Issue #125

Description

• treesitter now in Suggests
• namespace_function_calls() checks if treesitter is installed. Does not load the linter if it is not installed.
• The styling functions check if treesitter is installed. Does not run if it is not installed.

Definition of Done

• [ ] The change is thoroughly documented.
• [ ] The CI passes (R CMD check, linter, unit tests, spelling).
• [ ] Any generated files have been updated (e.g. .Rd files with roxygen2::roxygenise())

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 41.17647% with 10 lines in your changes missing coverage. Please review.

Project coverage is 97.21%. Comparing base (820e263) to head (7e4a45c).

Files	Patch %	Lines
R/namespaced_function_calls.R	14.28%	6 Missing :warning:
R/style_box_use.R	60.00%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #126 +/- ## ========================================== - Coverage 98.08% 97.21% -0.87% ========================================== Files 23 23 Lines 1096 1113 +17 ========================================== + Hits 1075 1082 +7 - Misses 21 31 +10 ```

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