Update landsat collections and adds function for using a custom image collection

[JonathanVSV]

Two main improvements were made to the script:

1. Updated Landsat collections (L5 - L9) to Level 2 Collection 2 Tier 1 surface reflectance.
2. Added a bfastMonitorCustom function that enables running the script with another image collection.

Other minor fixes:

1. Band names were replaced for letters, e.g., R, G, B, SWIR1, etc.
2. Added code examples in the README.
3. Changed mask to updateMask.

Remaining issues:

1. Some regions of interest (ROI) output the following error when showing or exporting the breaktime band: "Arguments must have same length along one axis". I could not fix that issue, but sometimes by just moving the ROI you can workaround the error.